Its the same old problem with zoonoses. Disease organisms evolve to parasitise particular species. Normally within that species they are relatively harmless, as to kill the host is very bad mannered. (just like dinner parties) Occasionally they will infect an animal that is not so comfortably evolved for them and then they tend to cause some real problems. An example is Bubonic Plague, which is a harmless parasite of mongolian marmots, causing the marmot equivalent of a mild cold. Once it gets into people or rats, however, its a swine.