Like many consumers in the natural world, parasites can be either specialists or generalists. One parasite may simply focus all their attention on one host species, or alternatively exploit a wide range of host species. Specialized viruses have the opportunity to really evolve closely with their host, and a good example would be the human chicken pox virus. This is an intriguing virus in the while this is really an airborne childhood disease transmitted by coughing, can also be transmitted through contact with a rash. It has evolved a persistence mechanism, which has allows it to become dormant in the body's nervous tissue, but then will reemerge when the host is much older and the immunity has waned at which point it causes the painful disease shingles in adults. Epidemiologically this is a clever strategy since all the children in an area recover from the infection and so carry antibodies preventing subsequent infection, the virus can still persist, and emerge later when these children become adults and they can, the adults can then initiate a new epidemic in children. Another infection of the nervous system that in contrast to chicken pox will infect does not infect a single host species but a wide range of host species is the rabies virus. And this will infect raccoons, skunks, foxes, coyotes, ferrets, bats, domestic cats, dogs, humans, and many, many other animals. The infection process at the cellular level for the rabies virus is similar, similar to many other species, and starts with the G-protein projections on the virus interact to the receptors on the host cell surface. The interesting feature about rabies is the viruses high affinity for nervous tissue, in particular that it binds to the acetylcholine receptors, which are not only expressed in high levels of nerve cells but are also highly consistent between many, many species. So the way these virus can infect the nervous system of many different animal species. On the one hand this provides flexibility, so the virus has more opportunities to infect different host species. But alternatively it does not have a within host presistent mechanism that we just heard about in chicken pox. So it's the fact that it infects multiple species, allows it to persist. Instead, it must invade many more species in the ecosystem, and in so doing attempt to persist over time. When parasites are driven from a tolerant host species to a vulnerable host species, then the more vulnerable host species can actually be eliminated through parasite mediated, or apparent competition. A nice example of this is in the United Kingdom, where American Grey Squir, Squirrels were brought over and released into the English countryside, and they have wiped out the resident red squirrels. The reason they've done this is because they both share a pox virus, but the greys are more tolerant of the pox virus infection than the reds. So the outcome is that the greys continue to spread the virus to the reds, and the reds die out, and they've been effectively been wiped out from most of their distribution. Quite simply through the effects of the virus as an infective competitive weapon. This doesn't happen in specialist infections because this is similar to what has happened in the tolerant species. Here transmission is dependent on the density of the host population, and so more hosts get infected when density's high but less when density's low, such that below the threshold host density the disease effectively fades out.