Believe it or not, the world is crawling with microbes! Most of them are absolutely harmless and in fact vital to maintaining balance in Earth’s ecosystems. But some can make us seriously ill with infectious disease; these are called germs (or pathogens, to scientists) — and it’s important that we know what they are, where they live, how they thrive, what threats they pose, and how to protect ourselves from them.

Germs fall into four categories: Bacteria, Protozoa, Fungi, and Viruses.