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.

History Of Microorganisms

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."

Animalcules All Around

In a letter written to the Royal Society, van Leeuwenhoek described finding thousands of animalcules in his own mouth.

He scraped plaque from his teeth, mixed it with water and put it under a microscope. To his surprise, van Leeuwenhoek wrote, "there was such an enormous number of living animalcules here, that I imagined I could see a good one thousand of 'em in a quantity of this material that was no bigger than a hundredth part of a sand-grain."

Van Leeuwenhoek assured his readers that he cleaned his teeth regularly but still, he wrote, "all the people living in our United Netherlands are not as many as the living animals that I carry in my mouth this very day."

Disease And Animalcules

He, himself, did not make any connection between his animalcules and the cause of disease, but before he died, other scientists had suggested that tuberculosis was caused by animalcules like the ones van Leeuwenhoek had observed.