Viruses: Microorganisms that invade living cells and need a host to survive and replicate. Viruses are responsible for many diseases that have no known cure, like the common cold and stomach flu. Antiviral drugs, like Tamiflu, which treats influenza, are difficult to develop, and few exist.
Bacteria: Tiny single-cell life forms that reproduce easily. Bacteria live in food and water and can survive on humans, too. Strep throat and E. coli are examples of bacterial infections.
Parasites: Organisms that live in or off a host, feeding off its tissues and fluids. Giardia, a common parasitic infection in the United States, spreads through contaminated drinking water and can cause diarrhea and cramps.
Fungi: Organisms, including yeasts, that can live in air, water, soil, and plants. Fungi can cause irritating infections and allergies, such as candida and athlete’s foot.