The emergence of 2009 H1N1 pandemic influenza

The emergence of a novel H1N1 virus in Mexico and the USA in spring 2009 and its rapid spread around the globe has led the World Health Organization to declare the first pandemic of the twenty-first century. Employing almost real-time sequencing technologies and disseminating this information freely and widely has permitted the most intensive investigation of the origins and evolution of an influenza pandemic in the history of this disease. The small levels of sequence diversity of the first isolates permitted a realistic estimate of when the 2009 H1N1 virus first entered the human population. The rate of change in influenza RNA sequences permitted several groups to trace the origins of this virus to swine and a reassortment of North American and Eurasian swine influenza. These virus strains in turn have been traced back to swine, avian, and human virus reassortments occurring years ago in swine, all the way back to the 1918-1930 H1N1 viruses. The influenza virus sequence information spans the dimensions of time (90 years), space (locations all over the world), and hosts (birds, humans, swine, etc.). The high evolutionary rate of this virus and the growing amount of information is allowing researchers to follow its changes in the search for possible factors that could contribute to an increase in its virulence.