The main symptoms of influenza are fever, chills, headache, muscle aches, dizziness, loss of appetite, and tiredness. Symptoms may also include cough, sore throat, runny nose, nausea, weakness, and depression. In infants, influenza can look like sepsis (a severe, generalized bacterial infection).

The flu is often confused with the common cold, but flu symptoms tend to develop quickly and are usually more severe than the typical sneezing and congestion of a cold. Because the flu can lead to pneumonia and other life-threatening complications, it's important to treat the flu seriously.

Influenza is a respiratory illness. Symptoms of flu include fever, headache, extreme tiredness, dry cough, sore throat, runny or stuffy nose, and muscle aches. Children can have additional gastro-intestinal symptoms, such as nausea, vomiting, and diarrhea, but these symptoms are uncommon in adults. Although the term "stomach flu" is sometimes used to describe vomiting, nausea, or diarrhea, these illnesses are caused by certain other viruses, bacteria, or possibly parasites, and are rarely related to influenza.

If you get infected by the flu virus, you will usually feel symptoms 1 to 4 days later. You can spread the flu to others before your symptoms start and for another 3 to 4 days after your symptoms appear. The symptoms start very quickly and may include

• Headache
• Chills
• Dry cough
• Body aches
• Fever
• Stuffy nose
• Sore throat

Typically, the fever begins to decline on the second or third day of the illness. The flu almost never causes symptoms in the stomach and intestines. The illness that some people often call "stomach flu" is not influenza.