Novel H1N1 influenza: A virus responsible for a flu pandemic in 2009 that was originally referred to as "swine flu" because many of the genes in this new virus were very similar to influenza viruses that normally occur in pigs in North America. However, the virus is actually a novel influenza A (H1N1) virus. This virus first caused illness in Mexico and the United States in March and April, 2009 that spread to pandemic status over the following months. H1N1 flu is spread from person to person, unlike typical swine flu, although it is not clear how easily the virus is able to spread among people. As of August 2009, a vaccine is under development for the novel H1N1 flu virus.