There are two main types of influenza that cause infection: influenza A and influenza B. Influenza A usually causes a more severe illness than influenza B. Influenza occurs most often in winter and usually peaks between December and March. The influenza virus is unstable and new strains and variants are constantly emerging.

For most people, an influenza infection is just a nasty experience, but for some it can lead to more serious illnesses. The most common complications of influenza are bronchitis and secondary bacterial pneumonia. These illnesses may require treatment in hospital and can be life threatening, especially in the elderly, people with asthma and those in poor health.

Even in winters when the incidence is low, 3,000-4,000 deaths have been attributed to influenza. NHS service and business continuity plans are being updated and revised to cope with future influenza pandemics.