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Infection with the flu (influenza) virus produces a variety of symptoms after an incubation period of one to four days. The period when a person is contagious may vary greatly. Patients frequently begin to transmit the virus the day before symptoms start. After that, most adults will continue to be contagious for another five days. Children may continue to spread the disease for 10 or more days and patients with immunodeficiencies may transmit it for months after symptoms fade.  

While many of the flu's effects are felt throughout the entire body, others are local and relate to the virus reproducing in the lungs and other parts of the respiratory system. Signs and symptoms of the flu typically include:

• Fever
• Muscle pain
• Headache
• Chills and sweats
• Fatigue
• Cough
• Sore throat
• Runny or congested nose
• Lack of appetite

Many of the symptoms of the flu in adults are very similar to those of the common cold. However, the National Institutes of Health (NIH) has established a list of the symptoms that typically distinguish the two conditions:

Flu symptoms may be different in infected children. Instead of a headache, children often develop middle ear infections, nausea and vomiting. Fevers in young children with the flu also tend to be higher than those that occur in adults and may lead to seizures. Although uncommon, the flu in children has also been associated with Reye syndrome, a fast-acting and deadly disease related to the use of aspirin in children with viral infections.

Symptoms of the flu typically fade within three to seven days. Cough and discomfort associated with the flu may persist for longer than most symptoms lasting for two weeks or longer.

In some cases, flu symptoms may begin to fade and then develop into severe complications. These may include pneumonia, muscle tenderness (especially in the legs) and Reye syndrome.