These are small creatures (animals) which invade and infect (infest) the body. They may be either:

• External parasites which live on or in the skin, e.g. scabies and sandworms.
• Internal parasites which live in the body. Many internal parasites live in the bowel (i.e. intestinal parasites such as worms). Other parasites live in the blood (e.g. malaria) or other organs such as the bladder wall (e.g. bilharzia).

Children with AIDS may be infected with unusual parasites not normally seen in healthy children (e.g. Toxoplasmosis).