Influenza covers it's shell with two types of accessories: the H spike, blue, and the N spike, red. Here the flu particle is sliced open to show its genetic material. Centers for Disease Control and Prevention, National Center for Immunization and Respiratory Diseases hide caption

itoggle caption Centers for Disease Control and Prevention, National Center for Immunization and Respiratory Diseases

Kimberly Delp gives a flu shot to Carleen Matthews at the Homewood Senior Center in Pittsburgh, Pa., last September. Andrew Rush/AP hide caption

itoggle caption Andrew Rush/AP

Sonia Despiar, right, a nurse with Gouverneur Healthcare Services, injects Imelda Silva with flu vaccine on Friday, Jan. 11, 2013, in New York. At least 10 elderly people and two children in New York have died from the flu and hospitalizations are climbing as the illness hits every county in the state. Bebeto Matthews/AP hide caption

itoggle caption Bebeto Matthews/AP

When flu viruses (in red) accumulate an escape protein too quickly, they exit the cell nucleus (in blue) before they've made enough viral copies to spread the infection. Benjamin tenOever hide caption

itoggle caption Benjamin tenOever

People line up at a Duane Reade pharmacy in New York behind a sign announcing the recent flu outbreak. Andrew Kelly/Reuters/Landov hide caption

itoggle caption Andrew Kelly/Reuters/Landov

Physician assistants Scott Fillman (left) and Andrew Hunadi get ready to see patients with flu symptoms, in a tent erected just outside the emergency entrance at the Lehigh Valley Hospital in Allentown, Pa. Matt Rourke/AP hide caption

itoggle caption Matt Rourke/AP

Registered nurse Michelle Newbury and physician assistant Scott Fillman see patients Thursday in a tent set up for people with flu symptoms, just outside the emergency entrance at the Lehigh Valley Hospital in Allentown, Pa. Matt Rourke/AP hide caption

itoggle caption Matt Rourke/AP

Reaching for relief: A customer at a pharmacy in New York City was grabbing some medicine on Thursday. Andrew Kelly /Reuters /Landov hide caption

itoggle caption Andrew Kelly /Reuters /Landov

Shea Catlin, a nurse practitioner, doses out flu vaccine to give a shot at a CVS Minute Clinic in Arlington, Va., on Jan. 3. Barbara L. Salisbury/The Washington Times/Landov hide caption

itoggle caption Barbara L. Salisbury/The Washington Times/Landov