Except healthy qi in human body, sometimes there is unhealthy qi which human body should not exist. Evil qi is that can cause disease and it includes wind qi, cold qi, summer qi, damp qi, dry qi and fire qi. It is called six qi and six qi is qi of seasonal pathogens, it connects with climate closely. Except six qi there is miasma and malign qi and they are toxic gas of nature and infective materials of infection disease.

Evil qi is qi that begins disease and sometimes acts in meridians and collaterals. Meridians and collaterals are portal that evil qi invades human body and routes that evil qi harms viscera organs in human body.

Factors of beginning of disease enter into human body. First it invades skin, if defense qi is weak in meridians and collaterals of human body and can not resist exogenous evil, it will enters into tertiary collateral vessel which is at acra of meridians and collaterals. If it does not deal with exterior effusion and expelling pathogen, pathogenic factor will enter along meridians and pass collaterals and meridians to invade viscera then cause disease of viscera and bowels. Different parts of exogenous evil invading human body there are different symptoms. When pathogen of warm heat invades tertiary collateral vessel, it will be febricity and afraid of cold, absent sweating or hyphidrosis and hydrodipsia. When invades collaterals it will be headache, thin and white lingual fur, floating and rapid pulse. When invades meridians it will be febricity but not afraid of cold, yellow urine, hydrodipsia, copious sweat and surging and rapid pulse. If evil heat invades lung, lung qi will be effective.

Besides the above symptoms, there are symptoms of discomfort in thorax cavity, inhibited of lung qi, cough and pain of supraclaveicular fossa. Further it will invade stomach along general collateral which is distributed over lung and appear bitter taste of mouth, flank pain, sicchasia, constipation, abdominal fullness and distending pain, yellow and scorch coated tongue, replete and deep pulse. Soon meridians and collaterals that are distributed over spleen from stomach meridian will be heavy body and tired limps, sicchasia and apocleisis, slimy tongue fur, moderate pulse and appear symptom of deficiency of both qi and blood, then impact kidney and damage archaeus which cause death. If pathogenic factor invades certain meridian but not treat, it will invade certain viscus; meridians and collaterals which are distributed over certain viscus invade other meridians and viscera and cause diseases of diverse viscera. Invasion of pathogenic factor can cause surgical diseases; purulent diseases are caused by pathogenic factors which invade meridians and collaterals. The reason why exogenous evil can invade human body is that defense function of human body weakens, six combinations in regulation system of meridians and collaterals and spigot switch are out of order, defense qi is disturbance and pathogenic factor catches your opponent off-guard then cause diseases.