There are 3 types (A, B and C) of the influenza virus that can cause flu. Each has subtypical strain. H1N1 is a strain of the type A virus. H1N1 is what epidemiologists call a “triple reassortment” virus because it combines genes from different flu viruses, viral strains that normally affect pigs (swine flu), along with birds (avian flu) and human flu strains.