If future influenza viruses are similar to those you've encountered before, either by having the disease or by vaccination, those antibodies may prevent infection or lessen its severity.

A number of virus subtypes have appeared in humans since the global epidemic (pandemic) of 1918, which killed tens of millions of people.

There are three types of influenza viruses: A, B and C.

Human influenza A and B viruses cause seasonal epidemics of disease almost every winter in the United States.

emergence of a new and very different influenza virus to infect people can cause an influenza pandemic