A parasite lives in a close relationship with another organism, its host, and causes it harm. The parasite is dependent on its host for its life functions. For example, viruses are common parasites. The parasite has to be in its host to live, grow, and multiply. Parasites rarely kill their hosts. A common, well-known type of a parasite is a hookworm. It is possible for humans or their pets to get them. Hookworms attach themselves in the lining of the small intestine, and cause diseases, and malnutrition as well, as they eat the nutrients and keep them from going to the host.
Here, we'll be talking about a different type of parasite, called a parasitoid. The parasitoid Encarsia pergandiella is a tiny wasp that lays its eggs in developing whiteflies. The wasp larva eventually kills the whitefly, and then emerges as free-living adults. This is why it is said that the wasps are parasitic. If you have ever seen Aliens, the developing aliens are parasitoids. Parasitods are different from normal parasites because they kill their hosts when they develop.