What is Influenza (Flu)?


Flu is a contagious respiratory illness caused by influenza viruses that infect the nose, throat, and sometimes the lungs. It can cause mild to severe illness, and at times can lead to death. The best way to prevent flu is by getting a flu vaccine each year.

Flu Symptoms

Flu can cause mild to severe illness, and at times can lead to death. Flu symptoms usually come on suddenly. People who have flu often feel some or all of these signs and symptoms:

  • fever* or feeling feverish/chills
  • coughs
  • sore throat
  • runny or stuffy nose
  • muscle or body aches
  • headaches
  • fatigue (tiredness)
  • some people may have vomiting and diarrhea, though this is more common in children than adults.
    *It’s important to note that not everyone with flu will have a fever.

How Flu Spreads

Most experts believe that flu viruses spread mainly by tiny droplets made when people with flu cough, sneeze, or talk. These droplets can land in the mouths or noses of people who are nearby. Less often, a person might get flu by touching a surface or object that has flu virus on it and then touching their own mouth, nose or possibly their eyes.

Who is most likely to get sick with flu?

A study found that children are most likely to get sick from flu and that people 65 and older are least likely to get sick from flu. Median incidence values (or attack rate) by age group were 9.3% for children 0-17 years, 8.8% for adults 18-64 years, and 3.9% for adults 65 years and older. This means that children younger than 18 are more than twice as likely to develop a symptomatic influenza virus infection than adults 65 and older.

Period of Contagiousness

You may be able to spread flu to someone else before you know you are sick, as well as when you are sick with symptoms.

  • People with flu are most contagious during the first 3 days of their illness.
  • Some otherwise healthy adults may be able to infect others beginning one day before symptoms develop and up to five to seven days after becoming sick.
  • Some people, including young children and people with weakened immune systems, may be contagious for longer periods of time.

Onset of Symptoms

The time from when a person is exposed and infected with influenza virus to when symptoms begin is about two days, but can range from about one to four days.

Complications of Flu

Complications of flu can include bacterial pneumonia, ear infections, sinus infections and worsening of chronic medical conditions, such as congestive heart failure, asthma, or diabetes.

People at Higher Risk from Flu

Anyone can get flu (including healthy people), and serious problems related to flu can happen at any age, but some people are at higher risk of developing serious flu-related complications if they get sick. This includes people 65 years and older, people of any age with certain chronic medical conditions (such as asthma, diabetes, or heart disease), people with a body mass index (BMI) of 40 kg/m2 or higher, pregnant people, and children younger than 5 years.

Preventing Seasonal Flu

The first and most important step in preventing flu is to get a flu vaccine each year. Flu vaccine has been shown to reduce flu related illnesses and the risk of serious flu complications that can result in hospitalization or even death. CDC also recommends everyday preventive actions (like staying away from people who are sick (distancing), covering coughs and sneezes, frequent handwashing, and taking steps for cleaner air) to help slow the spread of germs that cause respiratory (nose, throat, and lungs) illnesses like flu. More information is available about core and additional prevention strategies.

Diagnosing Flu

It is very difficult to distinguish flu from other viral or bacterial respiratory illnesses based on symptoms alone. There are tests available to diagnose flu. More information is available: Diagnosing Flu.

Treating Flu

There are flu antiviral drugs that can be used to treat flu illness.

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More information is available: “Seasonal Influenza, More Information.”

Source: CDC Website: https://www.cdc.gov/flu/about/keyfacts.htm

Last Reviewed: March 22, 2024

Source: Centers for Disease Control and Prevention, National Center for Immunization and Respiratory Diseases (NCIRD)