Simple, single-celled microorganisms

The smallest type of pathogen

Organisms such as yeasts, molds, and mushrooms that grow best in warm, dark, moist areas.

A large and complex single-celled organism

The protective lining that covers any opening into the body

A type of lymphocyte that helps the immune system destroy pathogens

A lymphocyte that produces antibodies

A protein that attaches to the surface of pathogens or to the toxins produced by pathogens, keeping the pathogen or toxin from harming the body.

An injection that causes the body to become immune to an infectious disease; also called a vaccination.

A substance containing small amounts of dead or modified pathogens or their toxins that is injected during an immunization.