Influenza: The flu is a contagious respiratory illness caused by influenza viruses that infect the nose, throat, and sometimes the lungs. It can cause mild to severe illness, and at times can lead to death. The most common symptoms include: a high fever, runny nose, sore throat, muscle pains, headache, coughing, and feeling tired. Usually, the virus is spread through the air from coughs or sneezes. Influenza spreads around the world in a yearly outbreak.