Lecture II

The concept of infection, however, is none the less valid here. For any highly tuberculous individual affects his fellow beings: and if any person is exposed to the sphere in which the tuberculous patient lives, then it may happen that the effect turns again into a cause. I have often tried to illustrate the relationship between primary causes of a disease and infection in the following analogy. Suppose that I meet a friend of mine, whose relations with other people do not in general touch me. He is sad and has reason to be so, for he has lost one of his friends by death. I have no direct relationship with this friend who has died, but I become sad with him at his sad news. His sadness is, so to speak, first hand and direct; mine arises indirectly, communicated through him. Nevertheless, the fact remains that the mutual relationship between me and my friend provides the pre-condition for this “infection.”