Influenza (flu) is an acute respiratory infection caused by the influenza virus. Symptoms typically appear suddenly and may include:
* High fever
* Chills and shivering
* Muscle, joint, and body aches
* Severe headache
* Weakness, fatigue, and general exhaustion
* Sore throat
* Cough (often dry initially)
* Runny or stuffy nose
* Sneezing
* Chest discomfort or pressure
* Shortness of breath (in more severe cases)
* Decreased appetite
* Nausea and vomiting (especially in children)

Symptoms usually begin 1 to 3 days after exposure to the virus. Symptoms such as fever and muscle aches generally tend to decrease after the 4th day of the illness.