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. This 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 doorknob or toilet seat can live there for a short time. They'll die quickly unless a live host comes along.
When they've moved into someone's body, though, viruses spread easily and can make a person sick. Viruses cause 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 (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 with weak immune systems (from diseases like HIV or cancer), though, may get more serious fungal infections.
Protozoa (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 like plant and animal cells.
Protozoa love moisture. So intestinal infections and other diseases they cause, like amebiasis and giardiasis, often spread through contaminated water. Some protozoa are parasites. This means 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.