Khan Academy: Learn all about the flu, flu vaccines and how to prevent it!

The flu is caused and spread by a virus called influenza, which has proteins on its outer coat (think of a person wearing a jacket) called Hemagglutinin and Neuraminidase. The flu uses these proteins to enter and exit cells, and we actually name these proteins H and N (easy to remember) and number them to keep track of all the different types that we have found. The flu is a sneaky little bugger though, and can avoid our immune system by making subtle genetic mutations over time (drift) or shuffling up its genetic material completely (shift)!