of domestic pigs in the ecosystem of influenza A". In November 2009, a study was published in Virology Journal in which it was suggested that the virus may be the product of three strains from three continents that swapped genes in a lab or a vaccine-making plant, and subsequently "escaped". The study followed debate among researchers in May 2009, when the authors asked the World Health Organization (WHO) to consider the hypothesis. After reviewing the initial paper, WHO and other organizations concluded the pandemic strain was a naturally-occurring virus and not laboratory-derived. Contagiousness The virus is contagious with an R0 of 1.4 to 1.6 and spreads from human to human in much the same way as seasonal flu. The most common mechanisms by which it spreads are by droplets from coughs and sneezes of infected people, and also potentially touching a surface or the hand of a person contaminated with the virus and then touching one's eyes, nose or mouth. In 2009, the WHO reported that H1N1/09 seemed to be more contagious than seasonal flu. However, a New England Journal of Medicine report said the transmissibility of the 2009 H1N1 influenza virus in households was lower than that seen in past pandemics. The U.S. CDC recommended that people should wait at least a day after their fever subsides (usually 3–4 days after the onset of symptoms) before resuming normal activities, but it has been found that they can continue to shed virus for several more days. Virulence The virulence of swine flu virus is mild and the mortality rates are very low. In mid-2009, the U.S. CDC noted that most infections were mild, similar to seasonal flu, and that recovery tended to be fairly quick. The number of deaths was said to be a tiny fraction of the annual number of deaths from seasonal flu. However, comparisons of human fatality figures with seasonal influenza are prone to underestimate impact of the pandemic, and the pandemic H1N1/09 virus was in fact the dominant strain of influenza causing illness in the 2009/10 flu season. Research carried out at Imperial College London has shown that, unlike seasonal flu, H1N1/09 can infect cells deep in the lungs. Seasonal flu can infect only cells with receptor type a2-6 which are typically located in the nose and throat but H1N1/09 can also infect cells with receptor type a2-3. This may explain why some patients experience severe