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Flu Basics

Influenza is a contagious respiratory illness caused by a virus. It attacks the respiratory tract (nose, throat, and lungs) in humans.

Every year in the United States, 5 to 20 percent of the population will get the flu. Of those, more than 200,000 people will be hospitalized from flu complications and about 36,000 will die.

Anyone can become infected, but the elderly, people with weakened immune systems and people with chronic health conditions are more likely to become seriously ill or die from the flu or its complications. The most common complication is pneumonia.

The information below describes the common flu signs and symptoms, how it is spread and who is most at risk.

Spread of the Flu

Flu viruses spread in respiratory droplets when infected people cough and sneeze. The virus becomes airborne and can be inhaled by anyone nearby. It can also spread when a person touches an object that has been contaminated with droplets and then touches their own mouth or nose before washing their hands.

Studies show that most healthy adults may be able to infect others one day before becoming sick and up to five days after they first develop symptoms. Some young children and people with weakened immune systems may be contagious for longer than a week. That means that you can pass on the flu to someone else before you know you are sick, as well as while you are sick.

Symptoms of the Flu

Symptoms of influenza resemble those of a cold, but come on quickly and are more pronounced. A person with the flu may experience fever, headache, tiredness, dry cough, sore throat, nasal congestion and body aches. Children can sometimes suffer from nausea and vomiting, 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 other viruses, bacteria, or possibly parasites and are rarely related to influenza.

Risks of the Flu

For the vast majority of people, the flu is a mild illness that lasts only a few days and does not require any serious treatment. Some people get much sicker and may even need to be hospitalized. Those most at risk for serious illness include people over age 65, young children, pregnant women and anyone with serious long-term health problems. This includes people with asthma, heart disease, kidney disease, immune system problems, diabetes, and blood disorders. 

Pneumonia, bronchitis, and sinus and ear infections are three examples of complications from flu. The flu can make chronic health problems worse. For example, people with asthma may experience asthma attacks while they have the flu, and people with chronic congestive heart failure may have worsening of this condition that is triggered by the flu.

Severity of the Flu

Flu is unpredictable and how severe it is can vary widely from one season to the next depending on many things, including:
  • what flu viruses are spreading,
  • how much flu vaccine is available
  • when vaccine is available
  • how many people get vaccinated, and
  • how well the flu vaccine is matched to flu viruses that are causing illness.

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