Disease evils are the pathogens of Chinese medicine. They include the external evils wind, cold, summerheat, dampness, dryness, and fire, which are environmental qì that affect the body. They also include internal evils that arise from imbalances within the body and that mimic the external evils, as well as phlegm and static blood.

Affliction by a disease evil is characterized by certain signs, most of which can be explained by natural causality, since they arise after the initial exposure. Exposure to cold, for example, is naturally followed by cold sensations, a curled-up body posture that preserves bodily warmth, a pale complexion resulting from constriction of blood vessels, and copious clear urine that results from the absence of normal fluid loss through sweating. Even though there may be fever, which is attributed to the body’s efforts to fight the evil, it is less prominent than the sensations of cold. Summerheat naturally gives rise to fever and sweating that causes a decrease in the volume of urine and a deepening of its color. Dryness naturally gives rise to a dry nose, dry mouth, and dry throat.

Despite the prevalence of natural causality, numerous conditions appear to have been associated with disease evils at least partly by a process of analogy, magical causality, and the qì paradigm. Wind, dampness, fire, and phlegm are major cases in point. Whether analogies guided observations or merely provided the language to explain pathological processes is, of course, a matter of speculation.

Wind

Wind in the environment is moving air, which, in our modern scientific view, is naturally stopped by the skin. It can only affect the inside of the body indirectly by its temperature- and moisture-reducing effects on the skin. Despite this, Chinese medical understands wind as a qì that, being able to penetrate matter, can enter the body and disrupt functions. However, the only way in which its presence is detected in the body is by the qualitative analogy of symptoms to the environmental influence. The points of analogy are established very clearly:

Wind easily damages yáng regions. It affects the yáng (upper and outer) regions of the body, just as wind in the environment affects things in high, exposed places more than things in lower, protected places. Wind is a major cause of what we now call colds, flu, and upper respiratory tract infections in which there are upper-body signs such as nasal congestion and sore throat, as well as exterior signs such as heat effusion and aversion to cold. A statement similar in meaning is wind is light and buoyant, emphasizing that it tends to affect the exterior, giving rise to a floating pulse.

Wind by nature causes opening and discharge. It opens the pores and causes sweating, just as wind in the environment can blow open doors and windows as well as damaging irrigation and drainage structures.

Wind is mobile and changeable. It can come suddenly and disappear suddenly, giving rise to common colds that usually go as quickly as they come. It can affect different parts of the body successively, giving rise to wandering pain and itching in different places.

Wind by nature stirs. It can cause spasms that cause the body to move in abnormal ways, just as wind in the environment causes trees and plants to bend, sway, and flap. Severe wind can also cause paralysis, just as environmental wind can break the branches of trees so that they no longer bend and sway.

Wind can also arise internally when a yīn-yáng imbalance results in a surfeit of yáng qì. Internal wind is characterized by various forms of spasm or immobility and reflects an analogy between the effect of wind on trees in the environment and the effect of internal wind on sinews in the body. Wind stroke was originally attributed to external wind, but as empirical observation caused earlier theories to be questioned, it eventually became attributed to internal wind. Whether the cause is external or internal wind, the contribution of analogy to the formulation of wind a cause of the disease is the same.

Dampness

Dampness is a yīn evil that affects people living in wet or damp places or working in wet or damp environments. It can arise internally when the lung, spleen, and kidney fail to deal with ingested fluids adequately.

Dampness by nature is heavy and turbid. It makes the limbs and head feel heavy. Being heavy, it tends to flow downward to the lower body, giving rise to vaginal discharge and diarrhea. Being turbid, it gives rise to murky, dirty-looking excretions, such as thick eye discharge, turbid vaginal discharge, turbid urine, and turbid nasal mucus. In such cases, it is often referred to as damp turbidity.

Dampness by nature is sticky and stagnant. Once it lodges in the body, it sticks around for a long time and is hard to eliminate. When it affects the skin, it gives rise to persistent conditions such as eczema. When it clogs the stomach and spleen, it gives rise to fullness and distension with ungratifying defecation. When it gets into the joints, pain of fixed location arises. Affecting the large intestine, it causes sticky, mucous stool accompanied by tenesmus.

Fire

As a cause of disease, fire is heat in the environment or heat that develops from internal causes. The word fire is here used metaphorically, since it refers to an evil prevailing in the environment rather than actual fire, and although it may give rise to a higher body temperature, there is never any combustion. Yet the causes and effects of fire in the body closely match the causes and effects of real fire in the environment.

Fire by nature flames upward. It naturally affects the upper body. Fire is also the cause of highly inflamed sores. The notion of fire causing a sore on the body's surface may have been influenced by the eruption of lava on the earth’s surface or by the discharge of hot water from hot springs and geysers.

Fire can be caused by qì stagnation (qì depression), static blood, dampness, and food accumulations. Common to all these factors is the idea of things failing to move or accumulating. In particular, qì stagnation and food accumulation are associated with flatulence (intestinal gas). The attribution of pathological fire in the body to such factors may have been influenced by marsh fires. Marshlands are places of stagnant water and rotting plant material that produces gases such as methane and hydrogen sulfide, which can undergo spontaneous combustion. The ability of static blood and dampness to transform into fire may be an extension of this.

Fire gives rise to wind and can be intensified by wind. Extreme heat engendering wind is a pathomechanism whereby a condition of high fever from externally contracted heat can give rise to wind that manifests in severe spasm (as in child fright wind). The understanding of this process may have been influenced by the observation of wind created by forest fires, where the rising of hot air draws in air from the surrounding area. Wind and fire fanning each other is a condition in which externally contracted wind and fire exacerbate each other. This idea may have been influenced by the observation of fire giving rise to wind and by the knowledge that a fire can be intensified by fanning or the use of bellows.

Phlegm

Phlegm is closely related to dampness. Both are considered forms of water-damp (水湿 shuǐ shī). Phlegm is of interest here because a distinction is made between tangible phlegm, which is coughed up from the lung, and intangible phlegm which can travel around the body, giving rise to numerous diseases without necessarily being associated with expectoration. Phlegm diseases of any kind tend to be intractable and often characterized by a glossy slimy tongue fur and a stringlike pulse.

Intangible phlegm can gather in lumps in various parts of the body, giving rise to scrofula, goiter, breast lumps, and even certain suppurative lesions. It can cloud the orifices of the heart, giving rise to epilepsy, feeble-mindedness, and mania or withdrawal (癫狂 diān kuáng, various kinds of mental illness). It is also a major cause of dizziness. Since these conditions do not necessarily involve expectoration of phlegm, their attribution to phlegm rests on similarities in the tongue fur and pulse and on the analogy between the physical properties of phlegm and clouding of the spirit.