The influenza virus is a complex, constantly changing virus. This makes it possible for someone who has already been infected with influenza to become re-infected in subsequent years. These small changes are known as antigenic drifts. An antigenic shift may cause a pandemic if the virus is easily transmitted from person to person. Because of the changes in the influenza virus, immunity to flu is short-lived, and therefore large segments of the population are susceptible to influenza every year.
Influenza is a seasonal illness in temperate climates.
The flu season in the temperate zones is during the winter months - November to March in the northern hemisphere, and May to September in the southern hemisphere. A pie chart showing the influenza isolates from October 1997 to September 1998 and from October 1998 to September 1999 is presented in Fig. A viruses infect several different animals including pigs, horses, other mammals, and aquatic birds as well as humans, whereas influenza B virus only infects humans. From time to time, influenza A viruses in animals and birds jump species and infect humans. The virus that caused the pandemic of 1918 is believed to have originated in pigs, while the pandemics of 1957 and 1968 are believed to have originated in birds.