Viruses, bacteria and parasites are living organisms that are found all around us. They exist in water and soil, on the surfaces of foods that we eat and on surfaces that we touch, such as countertops in the bathroom or kitchen. Some bacteria live in our bodies and do not cause problems. Other kinds of bacteria (as well as parasites and viruses) can make us quite ill if they invade our bodies. Bacteria and viruses can live outside of the human body (for instance, on a countertop) sometimes for many hours or days. Parasites, however, require a living host in order to survive.
Bacteria and parasites can usually be destroyed with antibiotics. On the other hand, antibiotics cannot kill viruses. Children with viral illnesses can be given medications to make them comfortable, but antibiotics are ineffective against treating these infections.
Bacteria, viruses and parasites can cause a wide variety of illnesses, and can infect any of the organs of the body. Viruses are often responsible for respiratory illnesses (such as the common cold) and digestive illnesses (such as diarrhea). Bacteria can infect any part of the body, but often cause diarrhea when they invade the digestive tract.