What is the flu (influenza)? The flu is an illness you get from the influenza virus. It causes symptoms like head and body aches, sore throat, fever and respiratory symptoms, which can be severe. Flu is most common in winter months, when many people can get sick at once (an epidemic). NHS information about flu, including symptoms, how to treat it yourself and when to get help. Flu, also called influenza, is an infection of the nose, throat and lungs, which are part of the respiratory system. The flu is caused by a virus. Influenza viruses are different from the stomach flu viruses that cause diarrhea and vomiting. Most people with the flu get better on their own. The flu, also called influenza, is a respiratory infection caused by viruses. Each year, millions of Americans get sick with the flu. Sometimes it causes mild illness. But it can also be serious or even deadly, especially for people over 65, newborn babies, and people with certain chronic illnesses. What causes the flu? Not to be confused with Flue or Common cold. Influenza, commonly known as the flu, is an infectious disease caused by influenza viruses. Symptoms range from mild to severe and often include fever, runny nose, sore throat, muscle pain, headache, coughing, and fatigue. A resulting 12-point cold & flu severity index is further broken out into five levels, helping to paint a regional and national picture of influenza activity in the US. The map also provides breakouts for each condition on the national and local level and is updated regularly with season progress. Learn about identifying flu symptoms, how the diagnosis works, common complications, and more. Influenza (the flu) is a viral infection that causes fever and chills, sneezing, coughing and a sore throat. The flu can be serious for some people, but if you are otherwise fit and healthy you usually do not need to see a doctor. There are four types of the influenza virus (A, B, C and D) — influenza A and B cause seasonal epidemics every winter, per the CDC. The predominant strains that circulate in the U.S. are H1N1. Flu is a contagious respiratory illness that infect the nose, throat, and sometimes the lungs. Learn about the current flu season and find resources related to flu surveillance and past seasons. Learn more about who is at higher risk of developing potentially serious flu complications.