Conditions & Treatments


Influenza (or flu) is a highly contagious viral respiratory tract infection.

Influenza (Flu)

What is influenza (flu)?

Influenza (or flu) is a highly contagious viral respiratory tract infection. It usually starts quickly, with fever, muscle aches, sore throat, and a dry cough. People of all ages can get the flu. Although most people are ill with the flu for only a few days, some have a much more serious illness and may need to be hospitalized. Influenza can also lead to pneumonia and death.

Influenza viruses continually change (mutate). Because the virus changes, people can get the flu no matter what their age. The process works like this:

  1. A person infected with an influenza virus develops antibodies against that virus.

  2. The virus changes.

  3. The "older" antibodies no longer recognize the "newer" virus.

  4. The person becomes infected again.

The older antibodies can, however, give some protection against getting the flu again. Currently, 3 different influenza viruses circulate worldwide. Vaccines given each year to protect against the flu contain the influenza virus strain from each type that is expected to cause the flu that year.

What are the facts about the flu?

Although each flu season is different, 5% to 20% of the population will get the flu each year. Of those who get the flu, between 3,000 and 49,000 will die from it or from complications, with more than 90% of deaths occurring in people over 65. 

What causes the flu?

The influenza virus is generally passed from person to person through the air - when an infected person sneezes or coughs. But the virus can also live for a short time on objects like doorknobs, pens, pencils, keyboards, telephone receivers, and eating or drinking utensils. So, you can also get the flu by touching something that has been handled by someone infected with the virus and then touching your own mouth, nose, or eyes.

What are the symptoms of the flu?

The following are the most common symptoms of the flu:

  • Fever

  • Headache

  • Runny or stuffy nose

  • Cough

  • Severe aches and pains

  • Fatigue or feeling very tired

  • Sometimes a sore throat

What is the treatment for the flu?

Treatment may include:

  • Medications to relieve aches and fever

  • Medications for congestion and nasal discharge

  • Bed rest and increased intake of fluids

  • Antiviral medications. When started within the first 2 days of treatment, they can reduce how long you'll have the flu and the severity of symptoms.

How can the flu be prevented?

A new influenza vaccine is available each year. You should get the vaccine as soon as it is available in your area.

The Center for Disease Control and Prevention (CDC) recommends that infants over the age of 6 months and all children and adults get flu shots every year.

Some people are at increased risk of developing serious complications from the flu. It is extremely important that these people get the vaccine.  They include those with:

  • Long-term heart and lung conditions

  • Other serious medical conditions such as

    • Endocrine disorders, like diabetes

    • Kidney or liver disorders

    • Weak immune system from disease or treatment; for example those with HIV or AIDS or taking long-term steroids or medications to treat cancer

    • Blood disorders such as sickle cell disease

  • It is also very important that others that have an increased risk of being exposed to the flu or are around people with increased risk of complications get the vaccine. They are:

    • Health care providers and other staff that provide care in hospitals, nursing homes, hone health, and other facilities

    • Household members, including children or people in high-risk groups

Types of vaccines

The flu vaccine is available as a shot and as a nasal spray. Your health care provider will determine which vaccine is right for you.

The shot is available in a few different forms. There is a high-dose vaccine for those over 65 and a vaccine for those with egg allergies. It is safe for most people. Talk with your provider if you have had

  • A severe allergic reaction to a previous flu vaccine

  • Guillain-Barre syndrome (a severe paralyzing condition)

The nasal spray is recommended for people from 2 to 49 years old. It should not be given to adults who:

  • Are pregnant

  • Have weak immune systems

  • Have egg allergy

  • Will be in close contact with someone with a weak immune systems

  • Have taken antiviral medication in the past 2 days

Antiviral medications are also available to prevent the flu in people who have been in close contact with others with the flu. Contact your health care provider if you have been exposed to someone with the flu.

Following these precautions may also be helpful:

  • When possible, avoid or limit contact with people who have the flu or symptoms of the flu.

  • Frequent handwashing helps to lessen the risk of the risk of infection. Wash them well for 15 to 20 seconds.

  • Cover your nose and mouth when coughing or sneezing. It is best to use tissues. Then wash your hands.

How effective is the flu vaccine?

Vaccine effectiveness varies from year to year. Vaccine effectiveness also varies from one person to another, depending on factors such as age and overall health.

What are the side effects of the flu vaccine?

The flu vaccine is safe. Vaccine safety is closely watched by the CDC and the Food and Drug Administration (FDA), and hundreds of millions of flu vaccines have been safely given across the country for decades.

The flu vaccine can't give you the flu. The most common side effects from a flu shot are soreness where the shot was given and maybe a slight fever or achiness. The nasal spray flu vaccine might cause congestion, runny nose, sore throat, or cough. These side effects are mild and don't last long.

When should I get a flu shot?

The CDC recommends getting the flu shot every year, as soon as it becomes available in your community. Flu season can begin as early as October and most commonly peaks in the U.S. in January or February. The flu shot takes 1 to 2 weeks to become effective.

If I get the flu shot, can I still get the flu?

If you get the vaccine, it is still possible to get the flu. People who have had the flu shot tend to have milder symptoms if they do get the flu.

Is traveling safe during the flu season?

Because the flu is a highly contagious infection usually spread by droplets from by an infected person who is coughing or sneezing, travelers are at increased risk of getting the flu.

The CDC recommends that travelers have the flu vaccine at least 2 weeks before planned travel to allow time to develop immunity. 

Treatment Programs

Massachusetts General Hospital understands that a variety of factors influence patients' health care decisions. That's just one reason why we're dedicated to ensuring patients understand their diagnosis and treatment options. Because a single option might not serve all patients, we offer a wide range of coordinated treatments and related services across the hospital. Patients should consult with their primary care doctor or other qualified health care provider for medical advice and diagnosis information.

Select a treatment program for more information:


  • Pediatric Imaging
    The Pediatric Imaging Program at Massachusetts General Hospital Imaging specializes in ensuring the safety and comfort of child patients while providing the latest technology and the expertise of specialized pediatric radiologists.

The following related clinical trials and research studies are currently seeking participants at Massachusetts General Hospital. Search for clinical trials and studies in another area of interest.

MGH employee flu vaccine documentation incentive

MGH Hotline 11.19.10 Hospital employees are reminded to document in PeopleSoft before Nov. 30 whether they have received or declined the seasonal flu vaccine to be eligible for a $50 bonus, payable in December.

Intense flu season at the MGH

At the MGH, there has been a considerable increase in the number of flu cases in the past several weeks, with health centers, outpatient practices and the Emergency Department reporting seeing an additional 40 to 80 patients a day with flu-like illness.

Our commitment to quality and safety

Mass General ensures that our patients receive the highest quality and safest care possible. Learn about our performance, our improvement goals and how we compare to other institutions.