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The flu is a contagious respiratory illness caused by influenza viruses. It can cause mild to severe illness, and at times can lead to death. The best way to prevent the flu is to get a flu vaccine each fall when the vaccine is available.
Every year in the United States, on average:
Some people are at high risk for serious complications, such as older people, young children, and people with certain health conditions.
Influenza is a respiratory illness. Symptoms of flu include:
Children can have additional gastrointestinal 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.
In the United States, the peak of flu season can occur anywhere from late December through March. The overall health impact (e.g., infections, hospitalizations, and deaths) of a flu season varies from year to year. CDC monitors circulating flu viruses and their related disease activity and provides influenza reports each week from October through May.
The main way that influenza viruses are spread is from person to person in respiratory droplets of coughs and sneezes. (This is called droplet spread). 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.
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.
Yes. 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. Those aged 65 years and older and persons of any age with chronic medical conditions are at the highest risk for serious complications of flu.
It is very difficult to distinguish the flu from other viral or bacterial causes of respiratory illnesses on the basis of symptoms alone. A test can confirm that an illness is influenza if the patient is tested within the first two to three days after symptoms begin. In addition, a doctor's examination may be needed to determine whether a person has another infection that is a complication of influenza.
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 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 three to seven days after they first develop symptoms. Some children may be contagious for longer than a week.
Each flu season is unique, but it is estimated that, on average, approximately 5% to 20% of U.S. residents get the flu, and more than 200,000 persons are hospitalized for flu-related complications each year. About 36,000 Americans die on average per year from the complications of flu.