Pigs might fly.

T here is a phrase in the English language: ''pigs might fly''. The phrase is used, since pigs will never fly, as a metaphor for something that will never, or is highly unlikely to ever happen. It is perhaps somewhat ironic then that although the pigs are not flying it is due to the interaction between pigs, as the source, and aviation, as the vector, that the current influenza outbreak has spread so rapidly around the world. It is reasonable to ask how this has come about and what the implications are for the respiratory community. Three criteria must be fulfilled for a pandemic to occur. First, a novel virus, to which a significant proportion of the world population have no immunity, must evolve. Secondly, this novel virus must have the capacity to cause human disease. Thirdly, and most critically, this virus must be easily transmissible between humans. Recent experience with avian influenza viruses has shown the development of novel and virulent viruses, which although able to cause very severe disease in humans, do not have the capacity to be transmitted easily from person to person. Many will not realise that human cases of avian influenza are continuing infection is only occurring in those in direct contact with infected birds. The new H1N1 virus, that is believed to have evolved in pigs in Mexico, differs from the previous avian influenza viruses in that it is more easily transmissible between humans and this is why the current outbreak is occurring. Influenza viruses are adapted to a number of avian and mammalian hosts and reach their greatest diversity in birds where they probably originally evolved. Most influenza viruses are host-specific, partly due to molecular interactions between the virus and host, but also because interspecies contact is unusual, in contrast to within-species contact which is common and necessary for virus transmission. The main respiratory epithelial receptor responsible for initial binding of the viral haemagglutinin is different in humans (a2,6 glycosi-dic linkage receptor) and birds (a2,3 glycosidic linkage receptor), which partly explains species specificity [1]. Pigs are unusual in possessing both receptors in their respiratory epithelium. This means not only can they be infected by both human and avian viruses, but that both infections can occur simultaneously. This provides the ideal situation for genetic re-assortment of material from both viruses, leading to a completely novel virus. The 1918 pandemic showed that air travel is …