Influenza viruses are encapsulated negative-sense single stranded RNA viruses. There are three types, Influenza A, Influenza B and Influenza C.

Influenza A (FLU A) viruses are the most virulent of the three and are associated with acute respiratory infections of varying severity, ranging from asymptomatic infection to fatal disease. The influenza A virus can be subdivided into different serotypes based on antibody response. Several serotypes of influenza A have been confirmed in humans including H1N1, which caused the Spanish Flu pandemic in 1918, and the Swine Flu pandemic in 2009.

Influenza B (FLUB) viruses do not mutate as readily and there is only one serotype, it has a narrower host range and is not known to cause pandemics. Both Inf A and B display similar symptoms and both may be fatal. Symptoms may include fever, sore throat, cough, headache and myalgia. Complications are common and include primary influenza, viral pneumonitis, bacterial pneumonia and exacerbation of chronic conditions; the disease tends to be more severe in the elderly, infants and young children, and in immunocompromised hosts