Flu symptoms will show up anywhere from one to four days after infection. They often appear suddenly and can be quite severe. Flu symptoms generally last one to two weeks before subsiding.

Common Influenza Symptoms

The most common symptoms of the flu are:

While most symptoms will taper off one or two weeks after onset, a dry cough and general fatigue can last several more weeks.

Other Influenza Symptoms

Other possible symptoms of the flu include dizziness, sneezing, and wheezing. Nausea and vomiting are not common symptoms in adults, but they sometimes occur in children.

Read about other causes of dizziness and wheezing

Emergency Influenza Symptoms

Pneumonia is a common complication of the flu, especially for certain high risk groups (see Influenza Risk Factors). You should visit an emergency room immediately if you have symptoms of pneumonia, including:

  • A severe cough with large amounts of phlegm
  • Difficulty breathing or shortness of breath
  • Fever higher than 104°F
  • Acute chest pains
  • Severe chills

Influenza vs. the Stomach Flu

An illness commonly known as the "stomach flu" is actually a form of gastroenteritis, or irritation of the stomach lining. Gastroenteritis can be caused by any number of pathogens, including viruses, bacteria, and parasites. Prominent symptoms include mild fever, nausea, and diarrhea. The influenza virus does not typically cause nausea or diarrhea, except sometimes in small children. It is important to distinguish between the symptoms of regular flu and the stomach flu so you can get the proper treatment. Learn more about gastroenteritis.