Influenza Virus causes severe respiratory illness and fever and is associated with worldwide outbreaks called pandemics. Incidence of Influenza is seasonal with greater numbers of infections occurring in the winter. Annually Influenza is estimated to infect 1 billion people leading to 300-500,000 deaths.

The most severe Influenza pandemic occurred in 1918, infecting 50% of the population and killing an estimated 20-50 million people (more than the first world war). Scientists and Doctors fear that the emergence of a new aggressive strain of Influenza may once again lead to a pandemic of such devastating proportions. Researchers are therefore closely monitoring the spread of Avian Influenza (H5N1) among birds, which is associated with a high mortality rate when it infects humans.

Influenza Viruses have two types of protein on their surface that are critical to the viruses replication strategy. The first, called Haemagglutinin is involved in attachment and cell entry, while the second, called Neuraminidase helps the newly created virus particles leave the infected cell. This image shows the shape of the Haemagglutinin molecule. Immunity to Influenza Virus particularly targets the two surface proteins. To evade this immune response viruses have evolved differently shaped Haemagglutinins and Neuraminidases, scientists have given these different numbers. Different species of Influenza are named according to the types of proteins on their surface e.g. H1N2. Pandemics (World-wide epidemics) occur when a new species emerges that human immune systems have never encountered before. The seasonal nature of Influenza outbreaks is the result of more subtle changes to these proteins.