The term prevalence (or, the percentage of infected individuals in a
population) is the proper term to use. It is defined thus:
(number of infections/number of individuals examined)*100
Abundance, however, is a term that you may have heard but is of limited
use in respect to microparasites becuase it is defined as:
(number of parasites/total number of individuals examined)
or, mean intensity x prevalence.
This term is of limited use when you cannot determine the number of
parasites in a host, as is common in microparasites.
These, and other terms, can be found in Schmidt and Roberts (1989),
Foundations of Parasitology; the original publication Margolis et al.
(1982), The use of ecological terms in parasitology. J. Parasitol. 68:
131-133; and in an upcomming volume of J. Parasitol.
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