Human cells infected with influenza virus, black and white transmission electron micrograph (TEM). Influenza (flu) is a respiratory disease caused by infection with a flu virus seen here budding from the cell surface. Symptoms can range from mild to severe, including: a high fever, runny nose, sore throat, muscle pains, headache, coughing, and tiredness. Influenza spreads around the world every year causing seasonal outbreaks, resulting in about three to five million cases of severe illness and about 250, 000 to 500, 000 deaths. Magnification: x 8000 at 10cm