Parasitism

Parasitism is defined in different ways by different authors, usually reflecting their research interest and bias. Parasitism, as used here, is defined as a close association between two organisms in which one, the parasite, depends on the other, the host, deriving some benefit (usually food) from it without necessarily damaging it. Traditionally, fungi, bacteria, and viruses, many of which are parasitic, are studied by microbiologists, whereas parasitologists study protozoan and metazoan parasites. In this contribution, only protozoan and metazoan parasites as well as higher plants (angiosperms) are included. Several types of associations resemble parasitism in various ways and cannot always be clearly distinguished from it, either because of insufficient knowledge or because genuine intermediate forms exist. Such associations are discussed in the following.