Historical perspective--Emergence of influenza A (H1N1) viruses.

N Engl J Med 2009;361:279­85. Copyright © 2009 Massachusetts Medical Society. On April 17, 2009, officials at the Centers for Disease Control and Prevention (CDC) confirmed two cases of swine influenza in children living in neighboring counties in California.1 Here we take a perspective from systems biology to review the series of evolutionary and epidemiologic events, starting in 1918, that led to the emergence of the current swine-origin influenza A (H1N1) strain (S-OIV), which is widely known as swine flu. This article is one of two historical articles on influenza A (H1N1) viruses in this issue of the Journal.2 Our review focuses on the key steps that characterize this viral evolution (Fig. 1).