Respuesta :

One of the biggest concerns in doctors that I have seen from watching TV is that flu pandemics if left untreated can turn deadly not within a matter of days but within a matter of minutes. Hope this helped.
Mutation on the virus, once mutated vaccinations are no longer effective. More people who get vaccinated the less likely there is a chance of virus mutation. This is called herd immunity.

Can spread very quickly and sometimes the method of transportation is difficult to pinpoint, therefore more difficult to contain.