Pigs are notorious incubators of influenza viruses and can be infected with more than one type of virus from different species. This is why they are believed to be critical in the evolution of new influenza flu viruses that can jump the "species barrier" from animals into humans to cause pandemic flu.

Flu viruses infecting the same animal can, in effect, have a form of sex whereby viral genes are exchanged between different viruses to produce novel forms of flu that can emerge with potentially devastating effect for the human population.

The main danger of a new flu virus emerging from pigs is that it would not be recognised by the human immune system and so result in severe infections that could quickly spread from one person to another - existing flu viruses circulating in the population are by contrast usually relatively mild. Scientists fear that the strain of swine flu in Mexico which appears to have jumped the species barrier into humans may have developed an efficient form of transmission that enables it to pass quickly from one person to another.

Medical authorities are particularly concerned that there seems to be at least three outbreaks of the virus in different parts of Mexico - the capital, Mexico City, San Luis Potosi in central Mexico and Mexicali - which suggests that the infection can spread easily.

The impact of a pandemic caused by such a virus is difficult to predict because it depends on a number of unknown factors, such as the level of existing immunity or protection resulting from seasonal flu already present in the human population, and the virulence or lethality of the infection. The virus responsible for the outbreak in Mexico belongs to a variant of the H1N1 subtype of influenza A. This particular strain of the H1N1 virus has not been previously detected in either pigs or humans, according to the World Health Organisation (WHO).

The letters refer to the two main viral proteins - haemagglutinin (H) and neuraminidase (N) - which have subtly different structures between different strains of flu virus. The H5N1 strain of bird flu was seen as a potential threat of developing into a pandemic, and the H1N1 strain has already caused the biggest pandemic in history - the outbreak of Spanish flu in 1918 which killed an estimated 50 million people.

In addition to known swine-flu viruses, pigs can become infected with avian flu viruses as well as viruses that are known to cause flu in humans. This is why pigs can are believed to be "mixing vessels" whereby the genes of these viruses can merge or "re-assort" in infected cells to create new, more virulent strains of the influenza virus.

Another worrying aspect of the outbreak in Mexico is that is seems to affect young and otherwise healthy people, rather than the usual risk groups of the very old and infirm. This has a disturbing parallel with the worst flu outbreak in history, the Spanish flu pandemic of 1918.

For the moment, the WHO is urging countries to increase their surveillance procedures to detect any increase in the number of flu cases over and above what would normally be expected from seasonal flu, which is constantly circulating within the population. Further laboratory tests and genetic studies will be carried out on the virus in an attempt to assess its potential to cause pandemic flu. Other tests have already confirmed that the virus is susceptible to at least one of the major antiviral drugs n Tamiflu - stockpiled in preparation for a future pandemic.