Why has medicine been so slow to improve over the centuries?

Medicine was founded by Hippocrates in the 5th century BC. Doctors continued to study the Hippocratic texts into the 19th century, and many of the therapies, such as bleeding, purgatives and enemas, continued to be practised into the 20th. The standard Hippocratic account of disease was that it resulted from an imbalance of humours within the body. But this failed to explain how some diseases spread through populations at particular times. Among the earliest Hippocratic texts, Epidemics and On Airs, Waters, Places sought to explain this phenomenon.

In 1850 the London Epidemiological Society was formed. The governing assumption of most of its members was exactly the same as those of the authors of the Hippocratic texts or of the Renaissance physicians studied by Carlo Cipolla in his Cristofano and the Plague (1973): epidemics were caused by the two things members of a population had in common, air and water. The obvious conclusion was that infectious diseases could be traced to foul air (malaria, miasma) and foul water, and could be prevented by hygienic measures such as the emptying of latrines and the removal of slaughterhouses to the outskirts of towns.