Influenza, also called the flu, is a contagious respiratory illness caused by influenza viruses. Types of flu include seasonal, avian, H1N1, and haemophilus. The flu can cause mild to severe illness, and at times can lead to death. The best way to prevent influenza is by getting a flu vaccination each year. There are two main types of influenza virus: Types A and B. Influenza A and B viruses that routinely spread in people are responsible for seasonal flu epidemics each year. The emergence of a new influenza virus like H1N1 can result in a flu pandemic as occurred in the spring of 2009. Over the course of a flu season, different types (A and B) and subtypes of influenza A viruses can circulate and cause illness. The flu is different from a cold. The flu usually comes on suddenly and symptoms can include fever, chills, cough, sore throat, runny or stuffy nose, muscle or body aches, headache, fatigue, vomiting, and diarrhea. Most people who get the flu will recover in a few days to two weeks, but some people will develop complications that can be life-threatening and result in death. Flu complications include pneumonia, bronchitis, and sinus and ear infections. The flu can also make chronic health problems like asthma and congestive heart failure worse. Anyone can get the flu (even healthy people), and serious problems from influenza can happen at any age. However, those most at risk from serious complications include people over age 65, people certain chronic medical conditions (such as asthma, diabetes, or heart disease), pregnant women, and young children.