This article, published by the WHO, assesses the potential for a global pandemic of A(H1N1) influenza, or 'swine flu'. The authors outline the properties of influenza viruses that are needed to create a pandemic and discuss population vulnerability and pandemic severity. They highlight the populations at most risk, for example people with underlying conditions such as cardiovascular disease, and the role that nutritional status and the quality of health services can play in influencing a pandemic's severity.

In assessing the 2009 swine flu outbreak, they draw attention to the fact that mutations often occur in influenza viruses, which means that while the emerging virus may be mild, it could return in several months in a much more lethal form.

The authors say that A(H1N1) is a new influenza virus not previously seen in humans or animals. They suggest that it is more contagious than seasonal flu, but note that outside Mexico where the outbreak began, the virus is causing very mild illness in otherwise healthy people. They emphasise the risk to people suffering from other chronic diseases and note that the WHO estimates that 85 per cent of these people are in developing countries.