Apart from emphasizing the role of healthy qi in causing disease, TCM also pays great attention to the role of pathogenic factors. Generally speaking, people with insufficient healthy qi only have clinical symptoms when they have been attacked by pathogenic factors. Pathogenic factors may dominate the occurrence of disease under certain conditions, such as high temperature, high-pressure electric current, chemical poison, wound caused by gun and bite by poisonous snake, etc. Take pestilence for example. It is also an important element in causing disease, usually leading to widely spreading disease. These pathogenic factors are usually hard to avoid, even for those with superabundance of healthy qi. That is why TCM advocates the idea of "avoiding poisonous factors" in cultivating health.
The Nature of Pathogenic Factors and Occurrence of Disease
Pathogenic factors of different nature may lead to different diseases. For example, attack by pathogenic wind leads to wind syndrome, attack by pathogenic cold leads to cold syndrome, attack by pathogenic dampness leads to dampness syndrome and attack by pathogenic heat leads to heat syndrome. Pestilence is more serious than six abnormal climatic factors and often causes infectious disease marked by quick spreading, acute onset and severity of morbid condition.
Severity of the Attack of Pathogenic Factors and Occurrence of Disease
Generally speaking, light attack causes mild disease because the struggle between healthy qi and pathogenic factors is mild; serious attack causes serious disease because the struggle between healthy qi and pathogenic factors is serious. Take wind attack for example. It may lead to cold attack and wind attack due to different degree of the attack. If the attack is severe, it causes cold attack; if the attack is mild, it causes wind attack.
The Region Being Attacked and Occurrence of Disease
Invasion of pathogenic factors into different regions may cause different diseases. For example, invasion of pathogenic cold into the muscles and meridians may cause headache and pain of the four limbs; invasion of pathogenic cold into the lung may cause cough and asthma; and invasion of pathogenic cold into the stomach may cause stomachache and vomiting.