It's also interesting how strongly people feel about this topic when it comes up. For example, a local issue here for us is that the deer population is unsustainably large, and almost every year there is a local election to get permission for the local government to cull the deer population. Invariably, it always gets shot down (pardon the pun), because there are a large enough group of people who "feel bad" for the deer, and don't want them shot. As a result, hunting deer as a pastime has now become dangerous, because there is a prion disease that has spread to deer that can infect humans as well (similar to mad cow disease). Just another example of too many animals in too close proximity with too much inbreeding resulting in more diseases of increased severity as the laws of nature try to balance the books. It's actually not anything like "laws of nature", the explanation itself lies in the opportunistic nature of most pathogens. For a virus/prion/bacterium/etc, the more things you can infect (more population density), statistics say the more likely you are to reproduce. Probably both the benign and the malignant pathogens are reproducing at high rates if there is a "substrate" of organisms available to reproduce in, but it's only the bad ones that have a measurable effect, the benign ones just go about their life and nobody measures them because they don't have an effect in population or function of the host organism.