Influenza pandemics are rare, and only influenza A viruses are known to cause pandemics. They can occur when mutation results in a significant and abrupt change in the influenza A virus’s antigens – this is called antigenic shift. This can also occur when an influenza A virus from the animal population gains the ability to infect humans. Antigenic shift may produce antigens so different from those of currently circulating viruses that most people do not have any immunity against them.