The germ theory of disease, which holds that certain diseases are caused by living organisms, occurred to people thousands of years ago, but it was proved only in the 19th century.
In the western tradition, the Roman scholar Marcus Terentius Varro first laid out the germ theory in his book, On Agriculture, a practical guide published in about 36 B.C. In it, Varro advises the farmer against building near swamps because “certain minute animals, invisible to the eye, breed there and, borne by the air, reach inside the body by way of the mouth and nose and cause diseases that are difficult to get rid of.”
Varro was a prodigious scholar and well known public figure, and his works were highly influential. However, at least some of his contemporaries, apparently including the writer/philosopher/statesman Marcus Tullius Cicero, considered his germ theory a crackpot idea.
It is worth noting that the Atharva Veda, the first Indian book that addresses medical topics, includes a fairly detailed germ theory. The book identifies a number of living organisms that were deemed responsible for causing various diseases, and prescribes cures to kill the organisms. The Atharva Veda was written down about 200 B.C., but its ideas may date as far back as 1,000 B.C.