Viruses are diverse because they can mutate very fast and can mix genes. They sometimes pick up genes from their hosts, and they can swap genes with other viruses. Some viruses, including flu viruses, carry out a kind of mixing known as reassortment. If two different flu viruses infect the same cell, the new copies of their genes get jumbled up as new viruses are assembled.
This is called “antigenic shift”:
(Hat tip: GrrlScientist, creator of the wonderful blog carnival, Scientia Pro Publica, the third issue of which was published on Monday on Deep Thoughts and Silliness.)