Influenza virus is a circular virus. There are three types of influenza virus (A, B, C). Type A is connected to widespread epidemics (Spanish flue 1918-19) occurring annually. Pandemics are rare. Influenza type A has several subtypes, and these are classified according to antigenic properties on the surface of the virus glycoproteins (spikes), hemaglglutinin (H) and neuraminidase (N). Humans are the primary reservoir for human infections. Transmission mode is airborne, incubation period varies from 1-3 days. Clinical symptoms are high fever, muscle pain, headache, sore throat, general fatigue. Complications like pneumonia occur. Vaccine is available.