A protozoan is a tiny, single-celled organism that lives in water, soil, or inside other living things. Unlike bacteria, which are even simpler, protozoans have a more complex cell structure with a nucleus and can move around on their own.

Protozoans are everywhere: in puddles, oceans, damp soil, and even inside your body. Most are harmless or even helpful, but some cause diseases like malaria or amoebic dysentery. An amoeba is a famous protozoan that moves by stretching out parts of its body like reaching arms. A paramecium swims using thousands of tiny hairs called cilia that beat like oars.

Despite being just one cell, protozoans can hunt for food, avoid danger, and reproduce. Some eat bacteria, others consume algae, and a few even capture other protozoans. Scientists study them to understand how life works at its most basic level. When you look at pond water under a microscope, those strange swimming shapes are often protozoans living their complete lives in a single drop of water.