When Hippocrates (or perhaps a contemporary) wrote these words, some time after 430 BC, he and his colleagues could do little for either good or harm to sufferers from infectious disease. Indeed, they themselves were at particular risk. Thucydides, describing the so-called plague of Athens of 430 BC (probably not bubonic plague, but unidentified) in his History of the Peloponnesian War, writes that “mortality among the doctors was the highest of all, since they came more frequently in contact with the sick.”I am grateful to Prof. Malcolm Dando for helpful comments on an earlier draft of this article.