Type A reappears every two to three years and type B every four to five. The 1919 epidemic killed 40m people worldwide. The infection is spread by coughing and sneezing. As the virus enters the body they attack the cells that line the upper respiratory tract. Symptoms include chills, fevers and muscular aches.
Mortality is low among the young but older people are especially vunerable.
A 1999 outbreak in Britain left 20,000 people dead from related illnesses. Its ability to kill stems from the fact that the virus can mutate quickly, often producing new strains against which human beings have no immunity. There is evidence that new strains come from animals. The 1919 epidemic is traced back to pigs and the later pandemics from birds. No cure exists for influenza and the most commonly followed course of action is to develop a vaccination for the next anticipated strain.