External Pathogenic Factors
External pathogenic factors refer to the disease-causing elements that invade the body from the outside, either through the skin or through the mouth and nose, leading to external diseases. External diseases are a category of illnesses caused by external pathogenic factors, typically characterized by sudden onset, with initial symptoms often including chills and fever, sore throat, and joint pain. External pathogenic factors can be broadly divided into two categories: the Six Excesses (Liuxin) and Epidemic Pathogens (Liqi).
1. The Six Excesses (Liuxin)
(1) Basic Concepts of the Six Excesses
1. The Six Qi and the Six Excesses
(1) The Six Qi: The Six Qi, also known as the Six Origins, refers to the six normal climatic elements of wind, cold, summer heat, dampness, dryness, and fire. The changes in the Six Qi are referred to as the Six Transformations. These normal climatic changes are conditions for the growth of all things and are harmless to the human body. Due to the body’s adaptive mechanisms during life activities, it develops a certain capacity to adapt, allowing physiological activities to align with the changes in the Six Qi. Therefore, normal Six Qi generally does not easily cause illness.
(2) The Six Excesses: The Six Excesses refer to the six external pathogenic evils: wind, cold, summer heat, dampness, dryness, and fire. The shifts in yin and yang, the alternation of cold and heat, and climatic changes all follow certain rules and limits. If climatic changes are abnormal, leading to excessive or insufficient Six Qi, or if the Qi appears at the wrong time (for example, experiencing cold in spring when it should be warm, or heat in winter when it should be cool), or if climatic changes occur too rapidly (such as sudden cold or warmth), exceeding certain limits, the body may fail to adapt, resulting in disease. Thus, the Six Qi transforms from being harmless to harmful, becoming a pathogenic factor. The Six Qi that can lead to disease is referred to as the “Six Excesses”. While climatic changes are closely related to the occurrence of diseases, not everyone will fall ill due to abnormal climatic changes. Some individuals can adapt to these changes without becoming ill, while others cannot adapt and do become ill. For the former, the same abnormal climatic change is considered a Six Excess. Conversely, when climatic changes are normal, even in favorable conditions, some individuals with low adaptability may still fall ill. This shows that the Six Excesses exist objectively in both abnormal and normal climatic conditions. The determining factors here are the differences in individual constitution and the strength of the righteous Qi. Only when the body’s righteous Qi is insufficient and resistance decreases can the Six Qi become pathogenic and invade the body, leading to illness. In this sense, the Six Excesses are a category of pathogenic factors that can cause external diseases due to the disruption of the body’s relative dynamic balance by changes in the Six Qi. The “Six Excesses” are also known as “Six Evils”.
2. External Six Excesses and Internal Five Evils
External Six Excesses are the pathogenic factors of external diseases, referred to as external evils, while Internal Five Evils refer to the pathological changes caused by the imbalance of yin, yang, Qi, and blood in the organs, including internal wind, internal cold, internal dampness, internal dryness, and internal heat (fire), which belong to the category of pathophysiology. Although the clinical manifestations of Internal Five Evils are similar to the pathogenic characteristics and pathological responses of the Six Excesses, they are distinguished from the external wind, cold, dampness, dryness, and heat by the prefix “internal”, hence called “Internal Five Evils”. Internal Five Evils generally do not present with exterior symptoms, but rather manifest as either deficiency or excess, or a mixture of both. External Six Excesses acting on the body lead to dysfunction of the organs’ yin, yang, Qi, and blood, resulting in pathological changes, which often present with exterior symptoms and are mostly excess conditions. Pure summer heat evil typically does not present with exterior symptoms, but when combined with dampness, it is referred to as summer heat dampness, which does present with exterior symptoms. Only when the external evil directly invades can interior symptoms be observed.
External Six Excesses and Internal Five Evils are both pathogenic factors and pathological results; although they are distinct, they are closely related. The Six Excesses can harm the body, penetrating from the exterior to the interior, damaging the organs, which can easily lead to the harm of Internal Five Evils. Conversely, when the Internal Five Evils cause dysfunction in the organs, they can also easily become susceptible to the Six Excesses.
(2) General Characteristics of Disease Caused by the Six Excesses
1. Seasonality and Regionality
(1) The Relationship Between the Six Excesses and Seasons: Since the Six Excesses are primarily the excessive or insufficient manifestations of the seasonal Qi, they tend to cause seasonal diseases. For example, wind diseases are more common in spring, summer heat diseases in summer, damp diseases in late summer and early autumn, dryness diseases in late autumn, and cold diseases in winter. This is a general rule. However, climatic changes are complex, and different constitutions have varying sensitivities to external evils, so different types of external diseases can occur in the same season.
(2) The Relationship Between the Six Excesses and Environment: An unsuitable working or living environment can also lead to the invasion of the Six Excesses and cause disease. For instance, prolonged exposure to a damp environment often results in dampness-related illnesses, while working in high-temperature environments frequently leads to summer heat, dryness, or fire-related illnesses. Dry environments are often associated with dryness-related diseases.
2. Singularity and Combination: The Six Excesses can cause disease either individually or in combination. For example, cold evil can directly cause diarrhea, while diseases caused by the simultaneous invasion of two or more types of evils include wind-cold colds, damp-heat diarrhea, and wind-cold-damp bi syndrome.
3. Transformability: After causing disease, the Six Excesses can not only influence each other during the course of disease development but can also transform into different pathological properties under certain conditions. For instance, cold evil can become heat due to stagnation, prolonged summer heat can transform into dryness harming yin, and all Six Excesses can transform into fire, etc. This transformation is related to individual constitution; individuals with strong or weak constitutions, and the state of Qi and organs (cold or heat) can influence this. Therefore, when evils invade the body, they often transform based on the Qi of the organs. Individuals with yin deficiency are most prone to transform into dryness, while those with yang deficiency are most prone to transform into dampness. Additionally, the duration of evil invasion is also related; generally, when the evil is first felt, it is not easily transformed, but when it has been stagnant for a long time, it is more likely to transform.
4. External Invasion: Diseases caused by the Six Excesses often have a process of transmission from the exterior to the interior. The evils of the Six Excesses typically enter the body through the skin or mouth and nose, leading to disease. In the initial stage of disease caused by the Six Excesses, the main clinical features are chills, fever, thin white tongue coating, and floating pulse, known as exterior symptoms. If exterior symptoms are not resolved, the disease can penetrate from the exterior to the interior, progressing from superficial to deep. Therefore, diseases caused by the Six Excesses often have a process of transmission from the exterior to the interior. Even if the evil directly invades the interior without exterior symptoms, it is still referred to as an “external disease”. Thus, the Six Excesses are considered the causes of external diseases.
The nature and pathogenic characteristics of the Six Excesses in TCM are derived from observations of natural phenomena, abstracted and summarized. In addition to climatic factors, the diseases caused by the Six Excesses also include various biological (such as bacteria, viruses, etc.), physical, and chemical pathogenic factors acting on the body, leading to pathological responses.
(3) The Nature and Pathogenic Characteristics of the Six Excesses
1. Wind
(1) Natural Characteristics: Wind has the characteristics of lightness, dispersion, and movement, being the primary Qi of spring. Among the twenty-four solar terms, the four terms of Great Cold, Beginning of Spring, Rain Water, and Waking of Insects are dominated by wind Qi. Since wind is associated with wood Qi and corresponds to the liver, spring is referred to as the season of wind and wood. Although wind is the primary Qi of spring, it is present throughout the year and in all seasons. Therefore, diseases caused by wind evil are more common in spring but can occur in other seasons as well.
(2) The Nature and Pathogenic Characteristics of Wind Evil: Wind is characterized by lightness, rapid movement, and variability; when wind prevails, it causes movement, making it the leader of all diseases.
① Lightness and Dispersion: Wind is a yang evil, characterized by lightness and dispersion, with the ability to rise and spread outward. Therefore, when wind evil causes disease, it tends to harm the upper body and easily invades the skin and muscles. If the lungs are affected, it can lead to symptoms such as nasal congestion, runny nose, throat itching, and coughing. When wind disturbs the head and face, symptoms may include dizziness, headaches, neck stiffness, and facial paralysis. If wind evil invades the skin, symptoms such as aversion to wind and fever may appear. Due to its dispersive nature, wind evil can invade the skin, causing the pores to open and leading to sweating and aversion to wind.
② Rapid Movement and Variability: Wind is characterized by rapid movement and lack of fixed location. “Rapid movement” refers to the property of wind evil to move easily and unpredictably, leading to symptoms that can shift in location, such as in wind rash or urticaria, which can appear and disappear; or in wind bi syndrome, where joint pain migrates. “Variability” indicates that diseases caused by wind evil can change rapidly and have sudden onset, such as in wind rash or urticaria, which can appear intermittently, or in conditions like epilepsy or stroke, which can cause sudden loss of consciousness. In summary, diseases led by wind evil, whether external or internal, generally exhibit characteristics of rapid onset, frequent changes, and quick transmission.
③ Active Nature of Wind: “Active nature of wind” refers to the characteristic of wind evil to cause instability and movement. This is often manifested as dizziness, tremors, convulsions, and other symptoms, hence the saying “when wind prevails, movement occurs”. For example, in external heat diseases, “extreme heat generates wind,” and in internal injuries, conditions like “liver yang transforming into wind” or “blood deficiency generating wind” also exhibit signs of wind evil.
④ Wind as the Leader of All Diseases: Wind evil is the precursor of external pathogenic factors; cold, dampness, dryness, and heat often attach themselves to wind to invade the body. For instance, when combined with cold, it becomes wind-cold evil; with heat, it becomes wind-heat evil; with dampness, it becomes wind-damp evil; with summer heat, it becomes summer wind; with dryness, it becomes wind-dry; and with fire, it becomes wind-fire. Therefore, clinically, wind evil is frequently encountered and easily combines with other evils, hence it is referred to as the leader of all diseases and the foremost among the Six Excesses.
Wind corresponds to the liver. Wind is wood Qi, which connects to the liver. External wind evil can lead to symptoms such as stomach pain, abdominal distension, borborygmus, vomiting, and diarrhea, which are caused by wind evil harming the liver, as wood overacts on earth.
In summary, wind is the primary Qi of spring and corresponds to liver wood. Wind evil can cause a wide range of diseases and changes rapidly. Its specific characteristics include: ① Widespread Impact: It can affect any part of the body, from the head to the feet, externally on the skin and internally on the organs. ② Mediating Role: It can combine with cold, dampness, summer heat, dryness, and fire to cause disease. ③ Specific Pathogenic Characteristics: Wind diseases have rapid onset, short duration, and easily recognizable symptoms, such as sweating, aversion to wind, itching, and migratory pain. In clinical practice, if the onset occurs in spring and is clearly related to wind evil, the presence of wind evil should be considered.
2. Cold
(1) Natural Characteristics: Cold has the characteristics of coldness and condensation, being the primary Qi of winter. The four solar terms of Minor Snow, Major Snow, Winter Solstice, and Minor Cold are dominated by cold Qi. Cold is associated with water Qi and corresponds to the kidneys, hence winter is referred to as the season of cold water. Since winter is dominated by cold Qi, cold diseases are more common in winter, but they can also occur in other seasons. Sudden drops in temperature and inadequate protection against the cold can also lead to the invasion of cold evil and cause disease.
(2) The Nature and Pathogenic Characteristics of Cold Evil: Cold evil is characterized by coldness, stagnation, and contraction.
① Cold as a Yin Evil, Easily Damaging Yang Qi: Cold is a manifestation of yin, and its nature belongs to yin. Therefore, cold is a yin evil. Yang Qi can normally control yin, but when yin cold predominates, yang Qi is not only insufficient to dispel cold evil but is also harmed by it, hence the saying “when yin is strong, cold prevails” and “when yin is strong, yang becomes ill.” Thus, cold evil is most likely to damage the body’s yang Qi. When yang Qi is damaged, it loses its warming function, leading to obvious cold symptoms in the whole body or locally. For instance, if cold evil constricts the exterior, it can lead to symptoms such as aversion to cold, fever, and no sweating, known as “injurious cold”. If cold evil directly invades the interior, damaging the yang Qi of the organs, it is referred to as “internal cold”. For example, if it harms the spleen and stomach, it can lead to abnormal digestion and absorption, resulting in vomiting and diarrhea; if it affects the lungs and spleen, it can lead to cough and shortness of breath, with clear phlegm or edema; if it harms the spleen and kidneys, it can lead to cold limbs, low back pain, clear urination, loose stools, and abdominal edema; if heart and kidney yang is deficient, cold evil can directly invade the shaoyin, leading to symptoms such as aversion to cold, curled up posture, cold extremities, clear diarrhea, mental fatigue, and thin pulse.
② Cold Nature of Stagnation: Stagnation refers to the obstruction and blockage. The movement of Qi, blood, and body fluids in the human body relies on the warming function of yang Qi to flow smoothly. When cold evil invades the body, the meridians and blood lose the warming effect of yang Qi, easily leading to stagnation and blockage of Qi and blood, resulting in pain, hence pain is an important characteristic of diseases caused by cold evil. Pain caused by cold tends to lessen with warmth and worsen with cold; warmth promotes the flow of Qi and blood, alleviating or reducing pain. Cold predominates and causes pain, but pain does not necessarily mean cold. Due to the different locations of cold invasion, the symptoms vary. If cold invades the skin, it can cause severe pain in the head, body, and limbs; if cold evil directly invades the interior, it can lead to cold pain or cramping in the chest, abdomen, or intestines.
③ Cold Nature of Contraction: Contraction refers to the pulling and tightening nature. Cold evil has the characteristic of contraction and tightness. “Cold causes Qi to contract.” When cold evil invades the body, it can cause the Qi mechanism to constrict, the pores to close, and the meridians and muscles to contract, leading to symptoms such as stiffness and pain in the joints or limbs; if cold invades the meridians and joints, it can cause contraction and pain, making movement difficult or causing numbness; if cold evil invades the skin, it can cause the pores to close, leading to symptoms such as fever and aversion to cold without sweating.
Cold corresponds to the kidneys. Cold is water Qi, which connects to the kidneys. When cold evil invades, excessive cold water can lead to reduced urination and edema; excessive cold water can also suppress heart fire, leading to symptoms such as heart pain, palpitations, and cold extremities.
In summary, cold is the primary Qi of winter and corresponds to kidney water. Cold diseases are more common in winter but can also occur in other seasons. The pathogenic characteristics of cold evil include: cold as a yin evil, easily damaging yang Qi, leading to obvious cold symptoms throughout the body or locally. Cold predominates and causes pain, making pain an important characteristic of cold conditions. Cold causes Qi to contract, leading to symptoms such as closed pores, retraction of Qi, and tightness in the meridians, resulting in symptoms such as no sweating, tightness, pain, or difficulty in movement.
3. Summer Heat
(1) Natural Characteristics: Summer heat is a fire-related evil, being the primary Qi of summer. The four solar terms of Minor Fullness, Grain in Ear, Summer Solstice, and Minor Heat are dominated by summer heat Qi. Summer heat has a clear seasonality, primarily occurring between the summer solstice and the beginning of autumn. Summer heat is unique to summer, hence the saying “summer heat is an external evil, and there is no internal summer heat.” Diseases caused by summer heat can be classified into yin summer heat and yang summer heat. Yang summer heat occurs due to excessively high temperatures in the summer, prolonged exposure to the sun, or working in hot environments, while yin summer heat occurs due to overconsumption of cold foods, excessive exposure to coolness, or prolonged cold baths during the hot season. In summary, yin summer heat occurs when exposed to cold in summer, while yang summer heat occurs when exposed to heat in summer.
(2) The Nature and Pathogenic Characteristics of Summer Heat Evil: Summer heat is transformed from fire, primarily characterized by upward movement and dispersion, and often accompanied by dampness.
① Summer Heat is Inherently Hot: Summer heat is characterized by the intense heat of summer, with a strong fire nature, hence it is classified as a yang evil. When summer heat harms the body, it often presents with a series of yang heat symptoms, such as high fever, irritability, flushed face, restlessness, and a rapid pulse, known as heat injury (or summer heat).
② Summer Heat is Upward and Dispersive: Upward and dispersive refers to the tendency of summer heat to rise and spread. “Upward” indicates that summer heat easily invades the head and disturbs the heart, while “dispersive” indicates that summer heat can harm body fluids and deplete Qi. As a yang evil, summer heat tends to rise, leading to symptoms such as profuse sweating, which can deplete body fluids. Excessive sweating can lead to thirst, dry lips, and scanty, dark urine. In cases of severe sweating, Qi may also be depleted, leading to symptoms such as shortness of breath and fatigue, and in extreme cases, heat stroke may occur. Heat stroke accompanied by cold extremities is referred to as heat stroke with cold extremities. Summer heat can also provoke liver wind, leading to symptoms such as convulsions, stiff neck, and even opisthotonos, referred to as summer wind (or summer epilepsy). Summer heat not only depletes Qi and body fluids but can also disturb the heart, leading to symptoms of irritability and restlessness.
③ Summer Heat Often Accompanies Dampness: The summer season is not only hot but also often rainy and humid, leading to a combination of heat and dampness. The clinical characteristics of summer heat include not only fever and thirst but also symptoms of heaviness, chest tightness, nausea, and loose stools due to damp obstruction. Although summer heat and dampness coexist, summer heat is the primary concern, while dampness is secondary; it is not necessary for summer heat to always be accompanied by dampness.
Summer heat is the primary Qi of summer, and summer heat evil can be classified into yin summer heat and yang summer heat. The basic characteristics of summer heat evil include intense heat, depletion of yin, and depletion of Qi, often accompanied by dampness. Therefore, clinically, summer heat is characterized by strong heat, yin deficiency, Qi deficiency, and damp obstruction.
4. Dampness
(1) Natural Characteristics: Dampness is characterized by heaviness, stickiness, and a tendency to descend, being the primary Qi of late summer. The four solar terms of Major Heat, Beginning of Autumn, End of Heat, and White Dew are dominated by damp Qi. Dampness corresponds to the spleen earth. During the transition from summer to autumn, damp heat can rise, leading to the prevalence of damp diseases in late summer. Dampness can also arise from exposure to water, rain, or damp living conditions. Damp evil can occur in all seasons and tends to harm the body slowly and insidiously.
(2) The Nature and Pathogenic Characteristics of Damp Evil: Dampness is a yin evil that obstructs the Qi mechanism and easily damages yang Qi, characterized by heaviness, stickiness, and a tendency to descend.
① Dampness as a Yin Evil, Easily Obstructing Qi and Damaging Yang Qi: Dampness is similar to water, which is yin in nature. Damp evil invades the body, stagnating in the organs and meridians, easily obstructing the Qi mechanism, leading to abnormal ascension and descent of Qi. When dampness obstructs the chest, it can lead to symptoms of chest tightness; when it obstructs the spleen and stomach, it can lead to symptoms such as loss of appetite, abdominal distension, and loose stools. Since dampness is a yin evil, when yin predominates, it can lead to yang illness. Therefore, damp evil can easily harm yang Qi. The spleen governs the transformation and transportation of water and dampness, and being an earth element, it is particularly sensitive to dampness. Thus, damp evil invading the body can easily obstruct the spleen, leading to symptoms such as diarrhea, edema, and reduced urination. “When dampness prevails, yang is weak,” indicating that dampness, as a yin evil, can easily damage the body’s yang Qi. To alleviate the harm of dampness, methods to transform Qi and drain dampness should be employed to restore the flow of Qi and regulate the water pathways, allowing dampness to be expelled through urination, thus restoring yang Qi.
② Dampness is Heavily Murky: Dampness is a heavy and turbid evil. The term “heavy” refers to the quality of being heavy and burdensome. Therefore, damp evil can lead to clinical symptoms characterized by heaviness, such as a heavy head, body fatigue, and heaviness in the limbs. If damp evil invades the skin, it can lead to symptoms of heaviness and obstruction; if it stagnates in the meridians and joints, it can lead to numbness and pain. “Turbid” refers to the quality of being dirty and unclean. Therefore, damp evil can lead to symptoms of unclean secretions and excretions, such as facial blemishes, excessive phlegm, loose stools, and muddy urine.
③ Dampness is Sticky: “Sticky” refers to the quality of being viscous; “stagnation” refers to the quality of being stuck. The sticky nature of damp evil manifests in two aspects: first, the symptoms are often sticky and unsatisfactory, such as sticky stools, difficult urination, and viscous secretions; second, the course of the disease is often prolonged and difficult to resolve due to the sticky nature of dampness. For example, damp warmth is an external heat disease caused by damp heat evil. Due to the specific nature of damp evil, it often presents with slow onset, prolonged course, and difficulty in rapid recovery, as seen in conditions like damp eczema and damp bi syndrome.
④ Dampness Tends to Descend: Dampness is similar to water, which tends to descend, hence damp evil has a downward tendency, easily affecting the lower parts of the body. Symptoms often manifest in the lower body, such as edema, which is more pronounced in the lower limbs, as well as conditions like leukorrhea, muddy urine, diarrhea, and dysentery, which are often caused by damp evil descending. However, damp evil can invade the body from all directions, not just the lower parts. The saying “those harmed by dampness are first affected below” from the Su Wen indicates that dampness tends to descend, easily affecting the yin regions, which is one of its characteristics.
Dampness is the primary Qi of late summer and corresponds to spleen earth. Damp evil has the nature of obstructing the Qi mechanism and easily damaging yang Qi, characterized by heaviness, stickiness, and a tendency to descend. Therefore, damp evil manifests as obstruction of the Qi mechanism, weakness of spleen yang, and accumulation of water and dampness, leading to symptoms such as chest tightness, abdominal distension, limb heaviness, nausea, and diarrhea, as well as unclean secretions and excretions.
5. Dryness
(1) Natural Characteristics: Dryness is characterized by dryness, contraction, and cleansing, being the primary Qi of autumn. The four solar terms of Autumn Equinox, Cold Dew, Frost’s Descent, and Beginning of Winter are dominated by dry Qi. Autumn is a season of contraction, with clear and dry weather, leading to a lack of moisture, hence the prevalence of dryness diseases. Dry Qi is transformed from the dry heat of autumn and is considered a yang evil within yin. Dryness can be classified into warm dryness and cool dryness. Early autumn may still have residual heat from summer, and prolonged sunny weather can lead to warm dryness invading the body, while late autumn approaching winter can lead to cool dryness invading the body.
(2) The Nature and Pathogenic Characteristics of Dry Evil: Dryness leads to dryness and easily harms the lungs, which is the basic characteristic of dry evil.
① Dryness Harms Body Fluids: Dryness is the opposite of dampness; when dampness is absent, dryness prevails. Dryness is transformed from the dry and killing Qi of autumn, characterized by dryness and depletion. Therefore, dry evil is most likely to harm the body’s fluids, leading to various dry symptoms and signs, such as dry skin, cracked lips, dry throat, dry hair, and reduced urination.
② Dryness Easily Harms the Lungs: The lungs are the most delicate organ, preferring moistness and disliking dryness, hence they are referred to as the tender organ. The lungs govern Qi and control respiration, directly interacting with the external atmosphere and opening to the skin and nose. Dry evil often enters through the mouth and nose. Dryness is the primary Qi of autumn and corresponds to the lungs, hence dry evil is most likely to harm the lungs. When dry evil invades the lungs, it can damage lung fluids, leading to symptoms such as dry cough with little phlegm, or phlegm that is difficult to expectorate, or even blood-streaked phlegm, as well as wheezing and chest pain.
Dryness is the primary Qi of autumn and corresponds to the lungs. The most important characteristics of dry evil are its ability to harm fluids and its tendency to harm the lungs. Whether external or internal dryness, symptoms of dryness can be observed in the mouth, nose, throat, lips, as well as dry skin and hair.
6. Fire (Heat)
(1) Natural Characteristics: Fire is characterized by heat and is predominant in summer. The four solar terms of Spring Equinox, Clear and Bright, Grain Rain, and Beginning of Summer are dominated by fire Qi. Since summer is the season of fire, fire corresponds to heart Qi. However, fire does not have the same clear seasonality as summer heat and is not limited by seasonal climate.
(2) The Relationship Between Warmth, Summer Heat, Fire, and Heat: Warmth, summer heat, fire, and heat share similar properties but also have distinctions.
Warmth and Heat: Here, warmth and heat refer to pathogenic evils. Warmth is the gradual increase of heat, while heat is the extreme of warmth; the two differ only in degree, with no essential difference, hence they are often referred to together. In the study of warm diseases, the term warm evil refers to all warm and hot pathogenic Qi, without distinction in degree.
Summer Heat and Fire (Heat): Summer heat is the primary Qi of summer and is transformed from fire; thus, summer heat is a heat evil. However, summer heat is unique to summer and is purely an external evil, with no concept of internal summer heat. Fire (heat) does not have clear seasonality and includes high temperatures, scorching heat, etc.
Fire and Heat: Fire is the source of heat, while heat is the nature of fire. Fire and heat are both manifestations of excessive yang, hence they are often referred to together. However, there are still certain distinctions; heat is purely an evil Qi, with no concept of righteous Qi. Fire, on the other hand, refers to the righteous Qi of the body, known as “lesser fire”; it can also refer to pathogenic fire, known as “excess fire”. This is the main distinction between fire and heat. Generally speaking, heat is often associated with external pathogens, such as wind-heat, summer heat, and warm heat. Fire, however, often arises from internal imbalances, such as heart fire, liver fire, and gallbladder fire.
In terms of warmth, heat, and fire, although they are all manifestations of the same Qi, warmth can transform into heat, and heat can generate fire, indicating a certain degree of difference. Warmth is a mild form of heat, heat is an extreme form of warmth; heat is a gradual form of fire, while fire is the extreme form of heat.
(3) The Meaning of Fire: In TCM, fire can be classified into physiological and pathological, as well as internal fire and external fire.
① Physiological Fire: Physiological fire is a type of yang Qi necessary for maintaining normal life activities, residing within the organs and having warming and transformative effects. This beneficial yang Qi is referred to as “lesser fire”, belonging to the category of righteous Qi.
② Pathological Fire: Pathological fire refers to excessive yang, which depletes the body’s righteous Qi. This type of fire is known as “excess fire”. Pathological fire can be further divided into internal fire and external fire.
External Fire: This can arise from exposure to warm and hot evils; it can also arise from the transformation of external evils such as wind, cold, summer heat, dampness, and dryness into fire, known as “five evils transforming into fire”. Among the five evils, only summer heat is purely external fire, referred to as summer heat. The other evils, such as wind, cold, dampness, and dryness, are not inherently fire evils; they can transform into fire under certain conditions. First, they must be constrained to transform into fire. Wind, cold, dampness, and dryness must be constrained for a long time to transform into fire. For example, cold can transform into heat, extreme heat can generate fire, or dampness can transform into heat, leading to severe heat. Second, individual constitution plays a role; individuals with excessive yang or deficient yin are more prone to transform into fire. Third, the location of the evil invasion is also relevant. For instance, if the evil invades the yangming dry earth, it is more likely to transform into fire; if cold evil directly invades the spleen, it is also likely to transform into fire. Additionally, the ability of the five evils to transform into fire is related to treatment.
Internal Fire: This often arises from dysfunction of the organs, leading to imbalances in yin, yang, Qi, and blood. Extreme emotional states can also lead to internal fire, known as “five emotions transforming into fire”.
In TCM, fire is classified into righteous and evil types. The righteous fire is known as lesser fire, which can be further divided into “sovereign fire” and “ministerial fire.” “Sovereign fire” refers to the yang Qi of the heart, while “ministerial fire” refers to the yang Qi of the liver, kidneys, gallbladder, bladder, pericardium, and triple warmer. Among these, the yang Qi of the kidneys is also known as “mingmen fire” or “dragon fire”, while the yang Qi of the liver is referred to as “thunder fire”. “Sovereign fire” refers only to righteous Qi; if it becomes excessive, it is termed “heart fire blazing”; while ministerial fire encompasses both righteous and evil aspects, and when excessive, it is termed “ministerial fire in chaos.” Both “heart fire blazing” and “ministerial fire in chaos” belong to “excess fire”, which is considered evil Qi.
(4) The Nature and Pathogenic Characteristics of Fire Evil: Fire evil is characterized by burning, upward movement, depletion of Qi, damage to body fluids, and the ability to provoke wind and cause blood movement.
① Fire is Burning: Burning refers to the characteristic of fire evil to scorch and burn. Therefore, when fire evil causes disease, the primary pathological mechanism is excessive yang Qi, clinically presenting with high fever, aversion to heat, and a rapid pulse, which are characteristic of heat. In summary, diseases caused by fire evil are characterized by significant heat symptoms, such as fever and rapid pulse.
② Fire is Upward: Fire is a yang evil, characterized by upward movement. Therefore, fire evil tends to manifest in the upper body. For example, if heart fire is blazing, symptoms may include a red, painful tongue, oral ulcers, and sores; if liver fire is excessive, symptoms may include severe headaches and red, swollen eyes; if stomach fire is excessive, symptoms may include swollen gums and bleeding gums.
③ Depletes Qi and Body Fluids: Fire evil, when it steams internally, easily forces body fluids to leak out, leading to depletion of body fluids. Therefore, diseases caused by fire evil often present with significant heat symptoms, accompanied by thirst, dry throat, scanty, dark urine, and constipation. When fire is excessively strong, it can deplete the body’s righteous Qi, leading to a decline in physiological functions. Additionally, since Qi is generated from water, and water can transform into Qi, the depletion of body fluids can also lead to Qi deficiency. For instance, when fire is excessively strong, symptoms such as high fever, sweating, and thirst may be accompanied by fatigue and weakness.
④ Can Easily Generate Wind and Move Blood: Fire evil can easily provoke internal wind and cause blood to move erratically.
Generating Wind: When fire heat invades the body, it often scorches the liver meridian, depleting body fluids and leading to internal wind, known as extreme heat generating wind. The combination of wind and fire can lead to rapid symptoms, clinically presenting with high fever, confusion, convulsions, stiff neck, and opisthotonos.
Moving Blood: Blood tends to coagulate in cold and flow in heat. Fire evil can scorch the blood vessels, causing blood to flow rapidly and leading to various bleeding symptoms, such as hemoptysis, epistaxis, hematochezia, hematuria, and skin rashes, as well as excessive menstruation and uterine bleeding.
⑤ Can Easily Cause Abscesses: Fire evil that enters the blood level can accumulate locally, leading to necrosis and pus formation, resulting in abscesses. “Abscesses are primarily caused by fire toxins.” “Fire toxins” and “heat toxins” are common causes of abscesses, characterized by localized redness, swelling, and pain.
⑥ Can Easily Disturb the Spirit: Fire corresponds to heart Qi, which governs blood and houses the spirit. Therefore, when fire evil harms the body, it easily disturbs the spirit, leading to symptoms such as irritability, insomnia, restlessness, and even confusion and delirium.
In summary, fire can be classified into physiological and pathological types. The fire discussed in this section is pathological fire, also known as fire evil. Fire evil can be classified into external fire and internal fire. External fire often arises from external invasion, while internal fire often arises from internal dysfunction. Fire evil is characterized by burning, upward movement, depletion of Qi, damage to body fluids, and the ability to provoke wind and cause blood movement. Its pathogenic effects are widespread, with rapid onset and the potential to spread rapidly. Clinically, it presents with high fever, fluid depletion, Qi deficiency, wind symptoms, bleeding, and abnormal mental states.
2. Epidemic Pathogens (Liqi)
(1) Basic Concept of Epidemic Pathogens
Epidemic pathogens are a class of highly contagious evils, also known as pestilential Qi, epidemic Qi, toxic Qi, strange Qi, mixed Qi, and perverse Qi. Epidemic pathogens are transmitted through the air and contact. Epidemic pathogens differ from the Six Excesses in that they are not caused by climatic changes but are microscopic substances (pathogenic microorganisms) that cannot be directly observed by human senses, referred to as “toxic” evils. Epidemic pathogens enter the body through the mouth and nose, thus belonging to external pathogenic factors. Diseases caused by epidemic pathogens are characterized by severe contagiousness and are referred to as epidemics, pestilences, or plagues (or warm epidemics). Warm diseases differ from plagues in that warm diseases are a general term for various acute febrile diseases caused by external factors, which are not contagious or epidemic.
(2) The Nature and Pathogenic Characteristics of Epidemic Pathogens
1. Rapid Onset and Severe Condition: Epidemic pathogens are characterized by rapid onset, burning heat, and intense toxicity. Therefore, their pathogenic effects are characterized by rapid onset, fierce progression, severe conditions, variable symptoms, and quick transmission, often harming body fluids, disturbing the spirit, moving blood, and generating wind. The harm caused by epidemic pathogens resembles that of fire heat, presenting a picture of intense heat; however, the toxicity of epidemic pathogens is often more severe, frequently accompanied by damp toxins, toxic mists, and foul Qi, making their pathogenic effects more intense and dangerous, with a higher mortality rate.
2. Strong Contagiousness and Easy to Spread: Epidemic pathogens have strong contagiousness and can spread through various routes, including the mouth and nose, among populations. Epidemic pathogens can cause sporadic outbreaks or large-scale epidemics. Therefore, epidemic pathogens are characterized by strong contagiousness, widespread outbreaks, and high mortality rates. Examples include major head plague (caused by epidemic toxins, characterized by redness and swelling of the head and face or sore throat), toad plague (characterized by swelling of the neck and head, resembling a toad), epidemic dysentery, diphtheria, rotten throat, smallpox, cholera, and plague, which include many infectious diseases recognized in modern medicine.
3. Specificity and Partiality: Specificity refers to the specificity of the disease location and type caused by epidemic pathogens. Epidemic pathogens have a certain selectivity in their effects on the organs, leading to corresponding symptoms in different locations. Different types of epidemic pathogens cause different diseases, each with its own clinical characteristics and transmission patterns, known as “one Qi causes one disease.” Partiality refers to the selectivity of epidemic pathogens towards certain species. Some epidemic pathogens affect humans, while others affect animals. Those that affect humans do not transmit to animals, and vice versa. Even those that affect animals do not transmit between different species.
In summary, both the Six Excesses and epidemic pathogens belong to external pathogenic evils, with different natures and pathogenic characteristics. However, the diseases they cause are often characterized by heat, hence they are commonly referred to as external febrile diseases.