Bacteria

Bacteria are microscopic living organisms — so tiny that millions could fit on the head of a pin. They’re single-celled, meaning each one is just one cell doing everything it needs to survive on its own.

They’re everywhere: in the air, soil, water, on your skin, and inside your gut. Most are harmless or even helpful — the bacteria in your digestive system help you break down food. A smaller number cause infections, like strep throat or a UTI, which is when your body needs help fighting them off, usually with antibiotics.

The simplest way to think about it: bacteria are living things, just extremely small ones. Viruses, by contrast, aren’t even fully “alive” in the same way — bacteria actually are.