Only subtypes bearing H1, H2, or H3 as their hemagglutinin cause human flu epidemics, because only those spread readily from person to person. Pigs offer conditions intermediate between what a flu virus finds in people and what it finds in birds; therefore pigs get infected with both human subtypes and bird subtypes. When an individual pig is infected simultaneously with two viruses—one adapted to humans, one adapted to birds—the opportunity exists for reassortment between those two.
