Introduction

The word pandemic is used to describe a disease that is epidemic throughout the world at more or less the same time. The other criterion for defining a pandemic relates to the causative virus. A pandemic occurs when a completely new virus emerges—a virus that shows a more radical change (antigenic shift) than the change occurring continuously in influenza viruses (antigenic drift) and which is generally associated with more severe illness. Britain is used to experiencing flu in most winters, but these outbreaks are not pandemics because they are not consistently present in all countries at the same time and not caused by new virus.

Transmission

Not all flu viruses have the same transmission properties. The virus causing avian flu in poultry spreads by faeco-oral transmission. It is widely believed that humans contracting this condition have acquired it as a result of contact with infected poultry, either by airborne spread from birds or their faeces or by contamination during food preparation. Anxieties surrounding the H5N1 avian flu virus arise for three reasons: it has infected and caused serious illness in humans; further antigenic drift may increase the likelihood of spread to and between humans; antigenic shift (a change to the virus nucleus occurring as a result of simultaneous infection in one species with different strains of virus) may result in a virus transmissible between humans and with a capacity to initiate a new type of flu virus infection and series of annual epidemics.

People typically acquire flu by inhaling the virus or by being in direct contact with the respiratory tract secretions of people who are infected. The potential to infect …