The H5N1 avian influenza virus is mutating to evade immune defenses in mammals following prior infection or vaccination.

Very few humans have gone up against bird flu. But we've all dealt with seasonal flu for years. Some of our immune systems might be primed to fend off a worse case, research finds.

New research suggests that past exposure to seasonal flu viruses may help protect people from severe illness caused by the H5N1 avian flu virus. Scientists at the Perelman School of Medicine at the University of Pennsylvania found that older adults who were exposed to flu viruses circulating before 1968 were more likely to have antibodies that recognize H5N1.