A flu-type ailment may have been first identified in 400 BC in animals. It was not until the 15 th and 16 th centuries, however, that there was clear evidence of influenza spread among humans. The virus in animals undergoes genetic mutation allowing it to infect humans.
Pandemics occur when new strains of influenza virus infect humans.
Researchers discovered that the influenza virus was in the orthomyxovirus family. The 1933 viral strain was named 'influenza A' and that of 1940 'influenza B'. It wasn't until 1947 that 'influenza C' was discovered. The influenza is spread from person to person through mists or sprays of infectious respiratory secretions caused by coughing and sneezing. The virus is inhaled into the respiratory tract and cells that line the airways, ciliated respiratory epithelial cells, are infected.