Domesticated pigs have often been regarded as a mixing vessel for
influenza viruses and reassortment of the seven viral gene segments
presenting an opportunity for new human strains to arise. Until 2009,
however, swine influenza was not regarded as a significant cause of
serious disease in humans. Cases of human infection began to emerge
towards the end of April 2009 in what is normally regarded as the
influenza season in the northern hemisphere. Beginning first in
Mexico, the new virus subtype often referred to as ‘‘swine flu’’ by
the popular press, spread rapidly throughout the world in a matter
of weeks.