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Flu (Influenza): Symptoms & Signs

  • Medical Author: Melissa Conrad Stöppler, MD

The flu (influenza) is a viral disease of the respiratory tract. Characteristic symptoms are

Other symptoms can occur, like

Symptoms of the flu usually last for one to two weeks. If vomiting or diarrhea is severe, symptoms of dehydration may develop.

Other conditions can sometimes cause similar symptoms, leading people to wonder if they really have the flu or a different condition. Haemophilus influenzae is a bacterium that was incorrectly considered to cause the flu until a virus was demonstrated to be the correct cause in 1933. This bacterium can cause lung infections in infants and children and can sometimes cause sinus or other infections. Other bacterial or viral infections, including the common cold, may cause symptoms similar to the flu. Sometimes with a runny nose, sneezing, head congestion, chest discomfort, stuffy nose, and cough, it is hard to determine whether a cold or the flu is the cause of the symptoms, although the flu tends to produce higher fever than the common cold. Sometimes, allergies produce severe respiratory symptoms as well. Symptoms of certain complications of the flu, such as pneumonia, lead to similar symptoms.

Causes of the flu (influenza)

Influenza viruses cause the flu (influenza). Influenza viruses are divided into three types, designated influenza types A, B, and C. Influenza types A and B are responsible for epidemics of illness that occur almost every winter and are often associated with increased rates of hospitalization and death. Influenza type C usually causes either a very mild respiratory illness or no symptoms at all.

Related Symptoms & Signs


Nguyen, Hien H. "Influenza." Medscape. Nov. 22, 2016. <>.

Medically Reviewed by a Doctor on 4/25/2017


Which illness is known as a viral upper respiratory tract infection? See Answer
Medically Reviewed on 4/25/2017

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