The highest rates of infection usually occur in children who then infect the adults in the family and elderly relatives. The virus also spreads very easily in closed communities such as ships, barracks and residential homes. A subtypes, including those of avian or bird origin. Most people will have no immunity to the new form of virus that develops, and the scene is set for a pandemic. People may be partially immune to these minor changes and, if they do develop influenza, may only suffer from a relatively mild illness.
Antigenic drift occurs from season to season resulting in epidemics every year.
After infection with the influenza virus, the incubation period before symptoms develop ranges from 18 to 72 hours. Most people infected with the influenza virus are ill for between six and 10 days. They often need to rest in bed for three days and restrict their activity for five days. Some people take longer to recover and may feel lethargic or 'off colour' for a few weeks. However, some infected people do not develop any symptoms 1 .