@Aisling "The first bacterium was discovered in 1676, as far as I know, but back then, nobody knew what they could do," the father answered. "In 1858, Rudolf Virchow published his 'cellular theory', according to which every disease has its origins in a cell.
A few decades later, Louis Pasteur discovered why milk gets rancid: He discovered that microbes - those are really small creatures - are the reason. He experimented a bit and found out that heat could kill them, which is why we say that milk gets pasteurized. He also found out that microbes can be airborne, and thus that diseases can be spread through air.
At about the same time, Robert Koch discovered that those microbes weren't the consequence of diseases - which had previously been discussed - but their cause. He also discovered the pathogens of tuberculosis and cholera.
The one who discovered disinfection, by the way, was Ignaz Semmelweis, and he discovered it without knowing what caused diseases.
The first antibiotic was discovered by Bartolomeo Gosio. He published his papers in 1893 for the first time, but they were barely taken notice of..."