Discuss the range of pathogenicity associated with eukaryotic microbes.

Some eukaryotic microbes are routinely pathogenic for humans or other hosts, whereas others are completely free-living and nonpathogenic. (It should be noted, however, that the majority of eukaryotic microbes are nonpathogenic for humans.) Probably the most nonpathogenic group is the algae, which, because they are photosynthetic, do not have to rely on the tissues of a host for their nutrients. Fungi exhibit a moderate level of pathogenicity; about 30% of fungal species can produce diseases (mycoses) in a variety of organisms, including humans. Many of these pathogenic fungi exist in two forms, a condition known as dimorphism. As a group, the protozoa exhibit the most striking instances of pathogenicity. However, this is not due solely to their nutritional requirements; like the fungi, protozoa are largely chemoheterotrophic. Protozoa are often pathogenic because they need moist, protected habitats because they lack a cell wall. Living in the tissues of a host allows protozoa to meet this requirement.