Viruses are even smaller than bacteria. They aren't even a full cell. They are simply genetic material (DNA or RNA) packaged inside of a protein coating. They need to use another cell's structures to reproduce, which means they can't survive unless they're living inside something else (such as a person, animal, or plant).
Viruses can only live for a very short time outside other living cells. For example, viruses in infected body fluids left on surfaces like a countertop or toilet seat can live there for a short time, but quickly die unless a live host comes along.
Once they've moved into someone's body, though, viruses spread easily and can make a person sick. Viruses are responsible for some minor sicknesses like colds, common illnesses like the flu, and very serious diseases like smallpox or HIV/AIDS.
Antibiotics are not effective against viruses. Antiviral medicines have been developed against a small, select group of viruses.
Fungi (pronounced: FUN-guy) are multicelled, plant-like organisms. A fungus gets nutrition from plants, food, and animals in damp, warm environments.
Many fungal infections, such as athlete's foot and yeast infections, are not dangerous in a healthy person. People who have weakened immune systems (from diseases like HIV or cancer), though, may develop more serious fungal infections.
Protozoa (pronounced: pro-toe-ZO-uh) are one-celled organisms, like bacteria. But they are bigger than bacteria and contain a nucleus and other cell structures, making them more similar to plant and animal cells.
Protozoa love moisture, so intestinal infections and other diseases they cause, such as amebiasis and giardiasis, often spread through contaminated water. Some protozoa are parasites, which means that they need to live on or in another organism (like an animal or plant) to survive. For example, the protozoa that causes malaria grows inside red blood cells, eventually destroying them. Some protozoa are encapsulated in cysts, which help them live outside the human body and in harsh environments for long periods of time.