The answer is that the influenza virus is a slippery character. Some viruses barely change at all over time. The measles virus, for example, is "as stable as stone," said William Schaffner, chairman of the department of preventive medicine at Vanderbilt University.

Chicken pox, too, is consistent from year to year. But influenza is "genetically plastic," said Schaffner. It mutates all the time, and it can combine with other flu strains to regularly make new variants.

Those kinds of changes happen so frequently that the body's immune system won't necessarily recognize this year's iteration of the flu as a dangerous threat—even if you suffered from or were vaccinated against last year's version.

"It's like putting on a new sport coat every