First Season of 2009 H1N1 Influenza.

The 2009 H1N1 influenza virus is responsible for the first influenza pandemic of the 21st century. Within 5 months after its emergence in Mexico, it had affected more than 290,000 patients and caused at least 3486 deaths on 6 continents. This report provides information about influenza viruses, previous pandemics, and the virology, clinical features, and epidemiology of 2009 H1N1 influenza in order to elucidate the current pandemic and outline the questions that remain.