Bacteria are very, very tiny. So tiny, that millions of them can fit on the tip of your finger! Because they are so small, you can't see the billions of them in the air, on your skin, and even inside your body! Bacteria may be small, but many of them are quite tough and can live just about anywhere.
Bacteria often get a bad reputation because some of them cause disease. But many others are completely harmless. Some bacteria even do good things for us, such as turn milk into cheese!
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