Germs
The biological cause of illness was a mystery to humankind for centuries. The ancient Greek physician Hippocrates theorized that poor health was caused by "bad air." In a 1677 research paper, Dutch scientist Antoni van Leeuwenhoek observed tiny organisms, later identified as germs, in a droplet of water. Although scientists noted specific germs in the blood of people with certain diseases, they believed the microorganisms were a product of the illness rather than the cause. As microscope technology improved during the 19th century, scientists deepened their investigation into these microorganisms and their relationship with disease.