A host is a person who allows guests to share his home and food. Although most guests are pleasant, some can be very selfish. Those which take-all that they can from their host are called parasites. Many plants and animals are parasites which feed upon other living plants and animals. They accept food and a home from their host. In return they offer only sickness and disease.
Houses for people are large enough to permit many kinds of plants and animals to live together. In one house there may be ants, geraniums, dogs, molds, moths, and people. To most parasites the body of the host is as large as a building. Parasites are always smaller than their host. Many cannot be seen except under a microscope. Thus, many thousands of parasites may live in one host.
Parasites often choose to live inside other parasites. A virus may live in a bacteria, which lives in a worm, which lives in a dog. One dog may be host to many kinds of parasites.
The word parasite really means "alongside food." Parasitism is concerned mainly with the problem of obtaining food. Certain plants and animals have found it easier to become parasites than to compete for food. Organisms which feed upon other plants and animals do not always find quantities of food. Parasitism flourishes among viruses, non-green plants, and animal groups.
PARASITIC PLANTS AND ANIMALS
A true parasite feeds only upon living plants and animals. Plants which feed upon dead or decaying matter are called saprophytes, while animals which feed upon dead organisms are called scavengers. A few parasites, like the blue-green mold on the orange, are able to live upon either dead or living hosts. Green plants, which contain chlorophyll, like the mistletoe, are able to manufacture part of their own food. Since they cannot manufacture enough food, they become semiparasites upon larger plants.
