The rate of reproduction of the parasite in the host organism and especially the rate of production of the infectious stage of the parasite are extremely important parameters capable of affecting the efficiency of transmission of the parasite from one host to another. As the multiplication of the parasite has a negative effect on the viability of the host, the rate of multiplication is a subject of optimization rather than maximization. In this connection, it is usually stated in the biological literature that a well-adapted parasite does not damage its host much and thus there is a gradual reduction in the pathogenicity and virulence of the parasite during the coexistence of the parasitic and host species. However, the situation is somewhat more complicated. Adaptation of the parasite to the host population is related primarily to maximization of its infectiousness. Infectiousness, i.e. the ability to infect further individuals of the host species, pathogenicity, i.e. the ability to damage the health (vitality) of the infected host, and virulence, i.e. the ability to reduce the fitness of the host, are generally related in some way; however, this connection need not always be a close one (Combes 2001). Infectiousness need not always be correlated with the rate at which the parasite reproduces in the host and the kind of infectious stage it produces. In some cases, a smaller number of progeny with sufficient resources can infect a greater number of new hosts than a greater number of progeny that are less well equipped for life. The rate of reproduction is very frequently positively correlated with the pathogenic manifestations of parasitosis. Pathogenic manifestations of parasitosis mostly shorten the time of survival of the host. Consequently, a rapidly reproducing parasite is frequently capable of producing a smaller number of progeny during the lifetime of the attacked host than a parasite that reproduces more slowly.
Simultaneously, the pathogenic manifestations of parasitosis almost always reduce the fitness of the host. However, the correlation between pathogenicity and virulence can sometimes be very loose. For example, a number of parasites mechanically or hormonally castrate their hosts. These castrators reduce the fitness of their hosts to zero, without in any way reducing their viability. Cases have been described where redirecting the resources of the host from the sex organs to its somatic tissues even increased the viability of the host (see XIX.6.3).
In some cases, host individuals that are more sensitive to the pathogenic action of the parasite have greater inclusive fitness than more resistant individuals. The fact that they rapidly submit to infection means that they protect their relatives, bearing copies of the same genes in their genomes, against the parasite. At other times, pathogenicity allows the immune system of the host to successfully identify the parasite, so that individuals with clinical manifestations of parasitosis have, in fact, a better chance of recovering (and thus greater fitness) than infected individuals without clinical symptoms.
It should be borne in mind that phytopathologists and physicians (probably with the exception of epidemiologists) employ the term virulence in different ways. Phytopathologists understand virulence to be the ability of a parasite to infect a certain strain of host, while physicians see this as the level of pathogenic manifestations of the infection. For the evolutionary biologist, the term virulence has two equally legitimate meanings – from the standpoint of evolution of the parasite, this describes the rate of reproduction of the parasite within the host and, from the viewpoint of evolution of the host, this corresponds to the degree of reduction of the fitness of the host by the particular strain (species) of parasite (Poulin 1998; Combes 2001).