Influenza, often referred to as the flu, is a common, contagious respiratory illness. There are four types of influenza viruses: A, B, C and D. While influenza A, B and C viruses can infect humans, influenza D is believed to primarily affect animals such as cattle and pigs. Influenza C is rare in comparison to influenza A and B, which are the main sources of the “seasonal flu,” or the viruses that circulate in Canada and other countries each winter. Influenza A is also the source of flu pandemics. Canada has experienced five influenza pandemics since the late 19th century, in 1890, 1918, 1957, 1968 and 2009. In Canada, influenza causes an estimated 12,200 hospitalizations and 3,500 deaths each year.