Influenza, commonly called “the flu,” is a common disease that infects the respiratory tract (nose, throat, lungs). Unlike some other viral respiratory infections, such as the common cold, the flu can cause severe illness and life-threatening complications in many people – especially those with underlying medical conditions such as asthma.

Although the term “stomach flu” is sometimes used to describe vomiting, nausea or diarrhea, these symptoms are not commonly caused by influenza but by unrelated viruses, bacteria or possibly parasites.

Seasonal flu is a term used to describe one of the many combinations of influenza viruses that mutate and circle the globe each year. It’s called “seasonal” because it strikes most often during the fall and winter. In the U.S., flu season can begin as early as October and run through March. Most people gradually build up an immunity to the viruses in seasonal flu, assisted by annual flu vaccinations.

Pandemic flu refers to a global outbreak of influenza (flu), such as the 2009 one involving H1N1 influenza. During a pandemic, an unusual flu virus spreads quickly around the world because people have less immunity to it, either because they have not been exposed to it before or have not been exposed in a long time.

What are the symptoms of influenza?

Influenza (flu) is a respiratory illness. Symptoms often come on very quickly and can include:

  • fever

  • headache

  • extreme tiredness

  • dry cough

  • sore throat

  • runny or stuffy nose

  • muscle aches

Children sometimes have additional gastrointestinal symptoms, such as nausea, vomiting and diarrhea, but these symptoms are uncommon in adults.

Some of the complications caused by flu include bacterial pneumonia, dehydration and worsening of chronic medical conditions, such as congestive heart failure, asthma or diabetes. Children may get sinus problems and ear infections as complications from the flu. Generally, those aged 65 years and older and persons of any age with chronic medical conditions are at highest risk for serious complications of flu.

Is the flu contagious and how does it spread?

The flu IS contagious. The period when an infected person is contagious depends on the age of the person. Adults may be contagious from one day prior to becoming sick and for five days after they first develop symptoms. Some children may be contagious for longer than a week. The time from when a person is exposed to flu virus to when symptoms begin is about one to four days, with an average of about two days. 

The main way that influenza viruses are spread is from person to person. This can happen when droplets from a cough or sneeze of an infected person are propelled (generally up to 3 feet) through the air and deposited on the mouth or nose of people nearby. (This is called “droplet spread.”)

Though much less frequent, the viruses also can be spread when a person touches respiratory droplets on another person or an object and then touches their own mouth or nose (or someone else’s mouth or nose) before washing their hands.

How can I keep from catching the flu?

By far, the single best way to prevent the flu is to get a flu vaccination each fall — especially people at high risk for serious complications from the flu and those in close contact with them (including household members).

Influenza vaccine provides the best protection available from flu—even when the vaccine does not closely match circulating flu strains, and even when the person getting the vaccine has a weakened immune system. Vaccination can lessen illness severity and is particularly important for people at high risk for serious flu-related complications and close-contacts of high-risk people.

Read more: Antiviral Medicines

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