Influenza viruses are divided into types A, B and C, plus subtypes of A. Types A and B circulate in human populations and mutate constantly, resulting in the need for a modified vaccine every year. At times a new influenza virus appears to which nobody is immune because no one has previously been exposed to it. This is called an antigenic shift and it occurs at irregular intervals.
Some antigenic shifts result in local epidemics or global pandemics.
In 1976, a new influenza virus from pigs caused human infections and severe illness. A vaccine against swine influenza was developed and administered in some countries, although no pandemic in fact occurred. The most devastating of the 20th-century influenza pandemics was that known as the Spanish Flu which killed more than 20 million people around the globe between 1918 and 1920. Prior to this, the H5N1 virus was known to infect only various species of birds, including chickens and ducks. After the first human case occurred in May 1997, intensive surveillance revealed 15 additional cases by the end of 1997, all of them in Hong Kong, Special Administrative Region of China.