Human: What is the flu? Assistant: 
The flu is a respiratory illness caused by a virus. It can cause a range of symptoms, including fever, cough, sore throat, runny nose, body aches, headache, chills, and fatigue. It can also cause complications such as pneumonia, bronchitis, and sinus infections. The flu is usually spread through droplets from coughing or sneezing, and can be prevented by getting a flu shot.