Girolamo Fracastoro – from Verona, theorized that epidemics were caused by invisible “seeds”, spread by fomites (Latin for tinder), not themselves dangerous but able to enhance transfer of disease.
Cf Miasma theory (bad air – like “Malaria”).  Took another 100 years before Van Leeuwenhoek found microorganisms under his microscope, scepticism continued even through the 19^th century despite work of Semmelweis.  Bassi was the first to show a microorganism caused disease, a “vegetable parasite” (actually a fungus) was causing the death of silkworms and the collapse of the italian silk industry.  Eventually John Snow and Louis Pasteur.