Influenza pandemics occur when a novel influenza virus with surface
antigens [hemagglutination (HA) and/or neuraminidase (NA)] to
which the majority of the human population has little or no preexisting
immunity emerges in humans and is able to be transmitted efficiently
from person to person. A novel HA or NA can be introduced
into the human population either through the direct transmission of
an animal (for example, avian) influenza virus to humans, or through
reassortment between human and animal viruses, or between different
lineages of swine influenza viruses (