A microbe is anything too small to be visible to the naked eye. Two types of microbes are bacteria and viruses. You’re surrounded by microbes all the time, and normally co-exist peacefully. Some types help you, like the bacteria in your digestive tract that help break down food. There are, however, some types of bacterium and viruses that can make you ill.

Bacteria are single-celled organisms. There are thousands of types of bacteria, and they live virtually anywhere. Bacteria are much bigger than viruses. (But they’re all way too small for you to see.) Bacteria are much more complex than viruses. Bacteria have the tools to reproduce themselves, by themselves. They are filled with fluid, and may have threadlike structures to move themselves, like a tail.

Virus. A virus may have a spiny outside layer, called the envelope. Viruses have a core of genetic material, but no way to reproduce it on their own. Viruses cannot reproduce on their own. They infect cells and take over their reproductive machinery to reproduce.