Also in the eighteenth century, Antonie van Leeuwenhoek discovered the first microscopic organisms with a microscope. He was the first microbiologist. He saw some of the microscopic organisms that cause diseases, but he didn't know what they were. In 1700, Nicolas Andry thought that some of these microscopic organisms caused smallpox and other diseases. Over 100 years later, Agostino Bassi figured out what caused disease in silkworms. Ignaz Semmelweis was a doctor who figured out that if doctors washed their hands after touching dead people, that other patients wouldn't get sick. That was in 1847. It was about this time that sanitation and hand washing became popular with some, but not all doctors. Doctors began to use chemicals to cleanse wounds and clean their tools between each patient.