Protozoa

Protozoa are single-celled organisms found in most typical environments like soil and fresh water. They prey on bacteria and fungi, but can sometimes cause disease in humans.

Plasmodium, the organism causing the tropical disease malaria, is perhaps the best example. Trypanosomes are the protozoa causing the African sleeping sickness. And then there is Naegleria fowleri, also known as the "brain-eating amoeba"…