jan 1, 1775 - Percival Pott discovered cancer
could be caused by external factors
He found that he was seeing increased instances of scrotal cancer, with the vast majority of cases in chimney sweeps. He concluded that the soot which got under their skin through sores was the cause of the cancer. Until this time, cancer was thought of as being caused by the humours. This also meant that cancer was potentially preventable, and there was no need to purge the body of fluids. Although he didn't have the vocabulary to describe it, he had discovered carcinogens.
Added to timeline:
The Emperor of All Maladies cancer timeline