Because flu viruses mutate nearly once every reproduction cycle, no two people are made sick by precisely the same virus, as illustrated by this chart documenting swine flu cases among humans in early 2009.

The recent outbreak and sudden spread of a novel H1N1 influenza virus has caused a worldwide concern and has tested our ability to respond to major public health challenges. Significant scientific resources have been marshaled to discover the best possible responses against this novel swine origin influenza virus. A group led by Raul Rabadan at the Center for Computational Biology and Bioinformatics, and the Department of Biomedical Informatics at Columbia University has been studying the evolution of influenza viruses and the origins of flu pandemics by analyzing large data sets that contain genomic information.