On Jerry Coyne's blog today, I found this:
In the early 1670s, pioneer microscopist Antoni Leeuwenhoek peered into the tiny glass ball of his single-lens microscope and looked at a capillary tube that contained water and crushed pepper-corns. Leeuwenhoek was trying to discover why pepper was hot, and although he never found the answer to that question, he made a momentous discovery: microbes. To his amazement, the water was full of bazillions of tiny organisms, and there seemed to be no end to them...I have an older brother who's a doctor. I remember the day he came home from medical school after looking at water for the first time under an electron microscope. He said, "I am never going to drink water again!"