Influenza (Flu)

Influenza (flu) is a respiratory tract infection caused by a virus.  It is spread from person to person primarily through droplets in the air caused a cough, sneeze or talking.

About 5 to 20 percent of people in the U.S. get the flu each year. The flu usually starts abruptly, with fever, muscle aches, sore throat, and a cough.

The flu can make people of any age sick. Although most people are sick with the flu for only a few days, some have a much more serious illness. They may need to go to the hospital. The flu can also lead to pneumonia and death.

Those most at risk for developing complications due to flu are children younger than five; adults 65 years or older; pregnant women and women who are two weeks postpartum; and those whose health could be further compromised due to a chronic health condition such as asthma, diabetes, heart or other organ diseases.

The flu viruses continually change. There are two main types of flu, A and B, which are further divided by subtypes and lineages, respectively . Vaccines are developed each year to protect against the flu virus strain(s) expected to cause illness that year.


The flu is caused by a virus. Viruses are 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, phones, and cups or eating utensils. So you can also get the flu by touching something that has been recently handled by someone infected with the virus and then touching your own mouth, nose, or eyes.


Each person may experience symptoms differently. The flu is called a respiratory disease, but it can affect your entire body. People usually become very sick with several, or all, of the following symptoms:

  • Cough, often becoming severe
  • Extreme exhaustion
  • Fatigue for several weeks
  • Headache
  • High fever
  • Runny or stuffy nose
  • Severe aches and pains
  • Sneezing at times
  • Sometimes a sore throat
  • Vomiting and diarrhea

Fever and body aches usually last for 3 to 5 days, but cough and fatigue may last for 2 weeks or more.

The symptoms of the flu may look like other medical problems. Always talk your healthcare provider for a diagnosis.

Is It a Cold or the Flu?

Signs and Symptoms Influenza Cold
Symptom Onset Abrupt Gradual
Fever Usual; lasts 3-4 days Rare
Aches Usual; often severe Slight
Chills Fairly common Uncommon
Fatigue, weakness Usual Sometimes
Sneezing Sometimes Common
Stuffy nose Sometimes Common
Sore throat Sometimes Common
Chest discomfort, cough Common; can be severe Mild to moderate; hacking cough
Headache Common Rare


The flu is diagnosed based on your symptoms. Lab tests may be used to confirm the diagnosis, if necessary.


Specific treatment for the flu will be determined by your healthcare provider based on:

  • Your age, overall health, and medical history
  • Extent and type of flu, and severity of symptoms
  • How long you’ve had symptoms
  • Your tolerance for specific medicines, procedures, or therapies
  • Expectations for the course of the disease
  • Your opinion or preference

The goal of treatment for the flu is to help prevent or decrease the severity of symptoms. Treatment may include:

  • Antiviral medicines. They can reduce how long you’ll have the flu, but they can’t cure it. They have to be started within the first 2 days of the illness. These medicines do have some side effects, such as nervousness, lightheadedness, or nausea. These medicines are prescribed by a doctor.
  • Medicines. There are medicines for congestion and nasal discharge. You can also take medicine to relieve aches and fever. Do not give aspirin to children with fever. The drug of choice for children is acetaminophen.
  • Rest. Bed rest and increased intake of fluids.

Talk with your healthcare provider for more information.


The most common complication of the flu is pneumonia. It can also cause serious muscle and central nervous system complications. Of those who get the flu, between 3,000 and 49,000 will die from it or from complications. Most of these deaths happen in people ages 65 and older.


A new flu vaccine is made each fall. Everyone ages 6 months and older should get a flu shot each year. It is usually recommended for specific groups of people, as well as for anyone who wants to avoid having the flu.

The flu shot is safe. The CDC and the FDA closely watch vaccine safety. Hundreds of millions of flu vaccines have been safely given across the country for decades.

The flu shot can’t give you the flu. The most common side effects from a flu shot are:

  • Achiness
  • Low-grade fever
  • Soreness where the shot was given

If you have them at all, these side effects are usually mild and last a short time.

The effectiveness of the vaccine varies from one person to another. It can depend on factors such as age and overall health.

The following may also be helpful for preventing the flu:

  • When possible, avoid or limit contact with sick people.
  • Wash your hands frequently to reduce the risk of infection.
  • Cover your nose and mouth when coughing or sneezing to limit spread of the virus.

The flu causes complications that may develop into a more serious disease or become dangerous to some people. This includes older adults and those with chronic medical problems. Always talk with your healthcare provider to find out if you should receive the flu shot.

Although the flu shot is safe, some people should NOT be vaccinated. These include:

  • People who are allergic to eggs
  • People who have had a severe reaction in the past after getting the flu shot
  • People who are sick with a fever (these people should get vaccinated after they have recovered)
  • Babies who are age 6 months old or younger
  • People who have a history of Guillain-Barré syndrome, a severe paralyzing illness, after getting the 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, but flu seasons are unpredictable. The flu shot takes 1 to 2 weeks to become effective.

The CDC recommends that travelers have the flu vaccine at least 2 weeks before planned travel to allow time to develop immunity. Talk with your healthcare provider for more information.

When to Call a Healthcare Provider

For most people, the flu can be treated at home without treatment from your healthcare provider. However, if your condition or situation makes you more susceptible to complications from the flu, tell your healthcare provider when you suspect you have the flu. If your symptoms get worse or you have new symptoms, let your healthcare provider know.

Key Points

  • The flu is an easily spread viral respiratory tract infection.
  • The flu is caused by viruses that are generally passed from person to person through the air.
  • The flu is treated with bedrest, increased fluid intake, and medicines to treat discomfort and fever
  • Antiviral medicines taken within the first 2 days of illness can reduce the length and severity of the disease but does not cure it.
  • Getting the flu shot every year is the best prevention.

Next Steps

Tips to help you get the most from a visit to your healthcare provider:

  • Know the reason for your visit and what you want to happen.
  • Before your visit, write down questions you want answered.
  • Bring someone with you to help you ask questions and remember what your healthcare provider tells you.
  • At the visit, write down the name of a new diagnosis, and any new medicines, treatments, or tests. Also write down any new instructions your provider gives you.
  • Know why a new medicine or treatment is prescribed, and how it will help you. Also know what the side effects are.
  • Ask if your condition can be treated in other ways.
  • Know why a test or procedure is recommended and what the results could mean.
  • Know what to expect if you do not take the medicine or have the test or procedure.
  • If you have a follow-up appointment, write down the date, time, and purpose for that visit.
  • Know how you can contact your healthcare provider if you have questions.