ABSTRACT
Parasites can be classified into two broad groups: microparasites , which multiply within, or on the surface of, the host, and macroparasites, which grow in or on the host, but do not multiply. Parasite transmission can be either horizontal or, less commonly, vertical. Horizontal transmission may either be direct or indirect, mediated by a vector or an alternate host. Invertebrates and plants are also able to suppress infections, but by less sophisticated, and usually less specific mechanisms. Immunity results in a loss of potential hosts for microparasites, which accentuates their boom-and-bust strategy. Plants and lower animals may also exhibit elevated immunity after parasite attack, but without the sophisticated specificity of vertebrates. Social parasites ‘social parasites’ do not feed on the tissues of their hosts like true parasites, but gain benefit from their animal hosts by coercing them to provide food or other benefits.
