Viruses, bacteria, and parasites are living organisms that are found all around you. They are in water and soil. They are on the surfaces of foods that we eat. They are also on surfaces that you touch, such as countertops in the bathroom or kitchen. Some bacteria live in and on your body and don't cause problems. Other kinds of bacteria (as well as parasites and viruses) can make you very sick if they get inside your body. Bacteria and viruses can live outside of the human body (such as on a countertop) sometimes for many hours or days. But parasites need a living host to survive.
Bacteria and parasites can often be killed with antibiotics. But these medicines can't kill viruses. Children sick from a virus can be given medicines to make them feel better. But antibiotics don't fight viral infections.
Bacteria, viruses, and parasites can cause many illnesses. They can infect any organ in the body. Viruses are often the cause of respiratory illnesses, such as the common cold, and digestive illnesses, such as diarrhea. Bacteria can infect any part of the body. But they often cause diarrhea when they get into the digestive tract.