Flu viruses are constantly changing, producing new strains. Pandemics occur when a virus develops that is so different from previously circulating strains that few, if any, people have any immunity to it. This allows the virus to spread widely and rapidly, affecting many hundreds and thousands of people.
A new pandemic virus may emerge through the mixing (recombination) of human flu virus with an animal flu virus such as a bird (avian) flu virus. |