A huge part of the living world is too small to be seen except under a microscope, which magnifies everything tens or even thousands of time. Some of these microscopic creatures are harmful to us, causing diseases or making food rot. Others are useful, such as the yeast that help us make bread, and the bacteria that make yogurt and cheese.
What's living on you?
In the wild, most animals have fleas, lice and other small parasites living on their bodies. The same was true of our ancestors. A large, warm-blooded creature such as a human being is like a walking restaurant to smaller forms of life, and the meals are all free. Today we use soap and water to get rid of these small parasites from our bodies and our clothes, and if all else fails we use chemicals to kill them. But there are many microscopic creature sharing our lives that we do not even know about. Most of them do us no harm at all. Some are even beneficial, such as a bacteria which live on our skins and in our intestines.