This article, written by the WHO Rapid Pandemic Assessment Collaboration and published in the journal Science, examines the spread of A(H1N1) influenza, or 'swine flu', and assesses its potential to cause a pandemic.

Analysing surveillance data from Mexico, the authors suggest the geographical spread of swine flu will likely be comparable to other twentieth century pandemics, although the associated impact on human health is difficult to predict.

The authors suggest that the outbreak originated in mid-February 2009 in the village of La Gloria, Veracruz, where over half the population suffered acute respiratory illness. They calculate that the virus transmissibility — the number of cases that one case generates on average — is between 1.4 and 1.6, similar to the transmissibility of previous flu viruses including those that led to the 1918, 1957 and 1968 pandemics. The authors estimate that by 30 April 2009, 6,000–32,000 people will have been infected, with 0.4–1.4 per cent of cases being fatal.