Early History of the Concept of Disease

Many of the original ideas about the development of disease are based in the concepts of religion. The first mention of the spontanous generation of disease comes from the Artharvaveda, which explains that disease is indeed caused by living agents that can be killed by drugs.  The Cannon of Medicine is the next major work that deals with the origin of disease. The cannon proposes that disease is caused by an infection of the humors of the body by foreign earthy substances. While the Cannon acknowledged the existence of bacteria and viruses, it denied that this was the major source of disease.

sources:

http://www.sacred-texts.com/hin/av.htm