Frozen Evolution. Or, that’s not the way it is, Mr. Darwin. A Farewell to Selfish Gene.

However, parasites exploit their environment, which is generally very rich in sources of nutrients, only temporarily, sometimes even transitorily, in terms of their lifetimes. The parasite generally dies with the death of the attacked individual and, in the vast majority of cases, its death is accompanied by the extinction of the relevant infrapopulation of parasites (a population of parasites living in/on one host). While, for organisms living independently, the critical parameters determining fitness are generally the rate of reproduction or the economy of use of food sources, for most parasites, the ability to be transmitted from an infected host to an uninfected host is of key importance. A parasite has only a very limited ability to affect its fitness through reproducing within a single host. From the standpoint of fitness, the number of progeny is of key importance, but only if they manage to get into other individuals in the host population. Consequently, most adaptive traits that are encountered amongst parasites are in some way related to the transmission of the parasite in the host population.

            The effectiveness of transmission is characterized by the basic reproductive rate R₀, i.e. the number of individuals of the host species that are infected, on an average, by one already infected host in the population that has not yet been affected by the parasite, i.e. in a population of uninfected and not-immune individuals (see also XVII.5). In real populations there are, of course, individuals that have already been infected and immune individuals, so that the actual number of individuals infected from a single source, i.e. the actual reproductive rate, R, is usually much smaller. Its value can be calculated from the equation

where q is the fraction of susceptible individuals, i.e. not yet infected and not-immune individuals in the host population. For endemically occurring parasites, the actual reproductive rate is equal to unity in the long term, so that the number of infected individuals in the population remains constant over the long term. The actual reproductive rate is a very important parameter from the epidemiological standpoint. From the standpoint of study of microevolution in the parasite population, however, rate R₀ is of key importance. 

Optimization of the reproduction of the parasite, usually, although not necessarily, accompanied by optimization of pathogenicity and virulence, is a typical evolutionary adaptation permitting influencing of the effectiveness of transmission and thus maximizing of R₀.  The ability to form resistant latent stages is also common; these can survive in the infectious state in nature until they manage to enter a suitable host. This aspect also encompasses the frequently very complicated life cycles of parasites, which often include a number of intermediate host organisms, through which the parasite gets from one host to another. Other adaptive traits are related to the ability of parasites to adaptively change the traits and behavior of the host organism to assist in its reproduction and dissemination.