2. H5N1 is the avian influenza virus subtype of concern and there appears to be little human immunity to it. The predominant winter strain of human influenza is H3N2. Most adults have some partial immunity to this strain, which caused a pandemic in 1968 when it evolved from avian influenza.
3. A variant of avian influenza, which is a type of influenza virulent in birds. It was first identified in Italy in the early 1900s and is now known to exist worldwide.