“There are many different strains on influenza and many of them differ in crucial cell-surface markers glycoproteins such as hemagglutinin (HA) and glycoside hydrolases called viral nuuraminidase (VN). Their identities make up the H and N numbers commonly seen in influenza names, such as H1N1, H5N1 or H7N9. What makes influenza interesting is that they typically have a natural reservoir in some animal and can spread between that group.
Sometimes it can spread to other organisms and even humans. Normally, this might be very dangerous for the person becoming infected, but does not produce sustained human-to-human transmission. If specific genetic mutations or alterations occur, it may become able to produce a human epidemic or even a pandemic.”