An introduction to avian and pandemic influenza.

There are many types of influenza viruses, which cause illness in a variety of birds and mammals. New strains are constantly evolving, causing seasonal influenza epidemics in humans. This article provides information about influenza and influenza viruses, and the three influenza pandemics of the twentieth century. Pandemic influenza is differentiated from avian influenza, which is a viral disease that primarily infects birds. The current outbreak of avian influenza H5N1 in poultry flocks across the world is unprecedented in its spread. Human infection with avian influenza is rare and for most strains the symptoms are usually mild. A notable exception is HN51, where almost 60 per cent of the currently recorded 251 human cases have died. While the risk of a pandemic occurring in the current circumstances is unknown, there is a high level of concern worldwide.