Isolating Mechanisms — Definition & Examples

H5N1 is also known as the avian flu. The flu is very, very common in birds. This flu virus can cause severe disease in humans. Humans tend to catch this flu by being in contact with infected birds, but this happens fairly rarely. It is even rarer that humans pass this flu to other humans.

If you imagine this as a classical case of isolating mechanisms, where the host "birds" is one island, the host "humans" is another island, and the H5N1 virus is a group of animals getting stuck on one island or the other, what mechanism of speciation would explain a change in H5N1 that allows it to transfer between humans?