That's because there are many different strains of flu virus. All strains cause flu symptoms, but their surface structure varies. Flu viruses have a coating that is studded with two different types of protein spikes known as N and H antigens (or Neuraminidase and Hemagglutinin antigens, if you like tongue twisters). It is these antigens that your immune system recognizes as something to fight. One year, the viruses spreading across the globe may contain N1 and H1 antigens, so it takes a vaccine with this combination to teach your body to fight off that strain. Meanwhile, however, the virus infects animal hosts such as pigs, chickens and ducks, and in the process reshuffles its surface structures. Next year's global cycle might have N1 H3, or N2 H2, combinations that slip past your immune system.

That's why public health officials sample the emerging viruses in Asia before they reach America. When the "new" flu begins to spread, the latest vaccine formulation-loaded with several new combinations of antigens-has you covered. Last year's vaccine still protects you against last year's virus strain, but it will take a fresh stick in the arm to get you through this winter.

-This Know It All was provided by Dr. Janet Salzwedel, a biology professor at Adrian College who specializes in botany and microbiology.