Figure 3.

A schematic representation of the relationship between human viruses and viruses from other mammals. Human viruses are depicted as a subset of mammal viruses, only partially protected by a species barrier. There are frequent minor incursions of zoonotic viruses (small arrows), and many of these may not persist in human populations. Occasionally there may be a much more significant event (large arrow) whereby a mammal virus proves capable of einvolving adaptation to infect and transmit from humans. doi:10.1128/microbiolspec.OH-0001-2012.f3