From the earliest periods of civilization onward and throughout all phases of the development of medical science, it was commonly believed that diseases were external in nature. Upon entering the human body, they disrupted its normal function, caused pain, and eventually destroyed it completely. Even modern medical science, no matter how enlightened it claims to be, has not abandoned this basic belief. In fact, bacteriologists rejoice at every newly discovered bacillus as a further indication of the external danger to human health.
