For his discoveries of microscopic life, Antoni van Leeuwenhoek is remembered today as one of the great geniuses of science. Using microscopes he made himself, Antoni van Leeuwenhoek peered into exciting new worlds that no one knew existed before. Beginning in the 1670s, he discovered tiny, single-celled living things that he called little animals. His curiosity led him to examine lake water, moldy bread, and even the plaque build-up on his own teeth! Van Leeuwenhoek was also the first to see red blood cells and bacteria.