When you’re sick, you might casually say, “I’ve got a bug,” or “I think I’ve got some kind of a virus.” Today, most of us take for granted the existence of microorganisms — tiny living organisms too small to be seen with the naked eye. Theories about invisible particles causing disease go back almost two thousand years, but it wasn’t until the late 1600’s that scientists finally saw microorganisms under a microscope and could see that they were alive. The Dutch scientist Antony van Leeuwenhoek was probably the first person to describe the organisms that before had been invisible. Van Leeuwenhoek found these organisms in all kinds of places and called them, “animalcules.” In a letter written to the Royal Society, van Leeuwenhoek described finding thousands of animalcules in his own mouth.