parasitic

What kinds of relationship does biology recognize?

The boundaries are less firm than the table suggests. Many organisms living harmlessly on or in a person cause disease if defenses weaken or if they reach a site where they do not belong, and some parasites produce no symptoms in a well person while causing severe illness in someone whose immune system is suppressed. That is why the same organism can appear as a harmless finding in one case and as an opportunistic infection in another.

Where is the word used?

Parasitic infection names an illness caused by a parasite, whether a protozoan, a worm or an organism living on the skin. Parasitic disease covers the range of conditions such organisms produce. A parasitic life cycle describes the sequence of stages an organism passes through, frequently across more than one species. Obligate parasites cannot complete that cycle without a host at all, while facultative ones can live independently and turn parasitic only under certain conditions. The word is also used loosely outside medicine for any one-sided arrangement, a usage borrowed directly from the biological sense.

How is a parasitic infection identified?

The methods differ from those used for bacterial or viral illness, because the target is a larger organism with a life cycle of its own. Stool is examined under a microscope for eggs, cysts or the organism itself, often on more than one sample, since shedding is intermittent. Blood films identify parasites that live inside blood cells. Antibody tests show exposure, though they cannot always separate a past infection from a current one. Imaging locates organisms that form cysts or masses within organs. A raised eosinophil count in the blood points toward worm infection rather than a bacterial cause, though it is neither sensitive nor specific enough to stand on its own.