Four ways of becoming ill.

Illnesses are classified by whether or not etiologies are comprehensible or incomprehensible. The comprehensible diseases are divided into those due to four classes of etiologic agents: toxicity, heredity, infection and deficiency. Cooperations between two members of these classes are described to illustrate the concepts of hereditary intoxication, infectious intoxication, toxic deficiency, hereditary infection, hereditary deficiency and infectious deficiency. Examples in which cooperations among members of three or four classes of etiologic agents are required to produce illness also are provided. It is concluded that there are only four known ways of becoming ill and that cooperations can explain much variability of human disease.