Influenza, commonly known as "the flu", is an infectious disease caused by an influenza virus.[1] Symptoms can be mild to severe.[5] The most common symptoms include: high fever, runny nose, sore throat, muscle and joint pain, headache, coughing, and feeling tired.[1] These symptoms typically begin two days after exposure to the virus and most last less than a week.[1] The cough, however, may last for more than two weeks.[1] In children, there may be diarrhea and vomiting, but these are not common in adults.[6] Diarrhea and vomiting occur more commonly in gastroenteritis, which is an unrelated disease and sometimes inaccurately referred to as "stomach flu" or the "24-hour flu".[6] Complications of influenza may include viral pneumonia, secondary bacterial pneumonia, sinus infections, and worsening of previous health problems such as asthma or heart failure.[2][5]