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The temporary or permanent relationship between two different species, in which one, the parasite, benefits by obtaining food and/or shelter at the expense of the other, the host. Some parasites have little effect on the host, some cause serious diseases, and some kill the host. Ectoparasites live on the surface of the host, while endoparasites live either inside or between the host cells. An obligate parasite, e.g. Phytophthora infestans, which causes late blight of potato, can only grow on its host. However, a facultative parasite, such as the fungus Pythium, can exist by feeding in a different way after the death of the host. A species of parasite may live on one particular host or a number of similar hosts, or may alternate between two or more different species. Some flowering plants are partial parasites, being able to live independently but becoming parasitic in certain circumstances. For example, some members of the Scrophulariaceae, such as eyebrights (Euphrasia), will photosynthesize and live independently but when their roots come into contact with those of grasses they become attached and absorb food.
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