Bacteria, fungi, viruses, and protozoa are all microorganisms . They can survive anywhere in the world. Some live in the human body, and people and microorganisms generally co-exist peacefully-often in mutually beneficial ways. But, a microorganism can cause an infection if it invades a part of the body where it is not welcome and multiplies.

Protozoa are microorganisms that contain their genetic material in a nucleus separate from the rest of the cell. Best viewed through a microscope, protozoa can be thought of as a diverse group of organisms that does not fit into the plant, animal, or fungus kingdoms. Toxoplasma is one important protozoan parasite that causes an infection (toxoplasmosis) of particular concern to pregnant women. Because protozoa can be difficult to grow in lab culture, evidence of infection is often determined by testing the blood for antibodies against the protozoan.