Influenza

The flu.  Every year, it strikes fear in the population, affecting 60 million Americans yearly.  It's also a multi-million dollar industry, between flu shots, doctor visits/hospitalizations, disinfectants, and over the counter medications.
This year is one of the most virulent years for the flu, with some of the most intense flu rates of the decade.  you can see the flu's progression for this and previous seasons at http://www.google.org/flutrends/us/#US. Although flu levels are dropping slowly, all 50 states are at either high or intense levels of flu.  Flu levels can be separated by state, or even some major cities.
The biology of the flu is quite complex, although it is a relatively simple virus, not even technically alive.  The virus has 8 protiens (we have about 30000), which are both inside and on the viral coat.  For more into, visit http://www.virology.ws/influenza-101/, which is a great site run by a professor of virology at Columbia University (Medical School).
On a side note, I have recently moved to a new lab here at the U of U, where we are studying the flu and the possibility of using one of the coat proteins that doesn't undergo genetic shift (hemagluttanin) to create a universal flu vaccine.  There are lots of interesting atricles on this type of research, which goes on all around the world.