[Host spectrum and virulence]

In an anthropozoonoses distribution of an infectious agent population into several host species at each generation can be considered to result in a fragmented anthropozoonoses environment. Insofar as reproductive success may vary between host species, gene flow becomes asymmetric and a source-sink anthropozoonoses system may be created. Under these conditions optimal virulence may be occur in any host species so that the source host species exerts the strongest selective pressure on the infectious agent. In the sink host species the virulence of the parasite may be sub- or super-optimal (stable misadaptation). This explains the gravity of parasitic infections that man shares with animals and the fact that virulence does not diminish with time.