Two types of influenza A widely circulate in humans and cause seasonal outbreaks and epidemics - H1N1 (swine flu) and H3N2 (a subtype abundant in seasonal influenza). During the 2009-2010 influenza season, the most commonly reported virus was influenza A (H1N1). However, during the 2010-2011 influenza season, the most commonly reported virus was influenza A (H3N2), but influenza A (H1N1) viruses and influenza B viruses circulated as well.