Influenza is an infection caused by a virus. It can affect anyone of any age. It spreads rapidly from person to person when people are in large groups, such as at school, at work, in daycare centers or at public places or events. Flu usually hits between October and May, peaking in late December through early March.

The flu virus attacks a person's nose, throat and lungs. Although people sometimes speak of "stomach flu" when referring to diarrhea, nausea and vomiting caused by a virus, this is not true influenza.

Symptoms of flu can range from being as mild as the common cold to being fatal. Each year, more than 35,000 Americans die from influenza and 150,000 more require hospitalization.


Colds and influenza have some common symptoms, such as coughing and sneezing. Individuals with the flu, however, usually feel worse, take longer to fully recover and may experience other symptoms, such as:

  • Chills and sweats
  • Fatigue
  • Fever of 101 to 106 F
  • Fever that lasts up to a week
  • Loss of appetite
  • Muscle aches (especially in the back, arms and legs)
  • Sore throat
  • Stuffy nose
  • Weakness

Symptoms of the virus usually appear one to four days after exposure to the virus. Individuals generally feel better in one week to 10 days. Flu can lead to serious lung infections, such as pneumonia or swelling of the lining of the airways in the lungs (bronchitis).

Causes and Risk Factors

Influenza is caused by three types of viruses:

Influenza A, which was responsible for the deadly worldwide epidemic in 1918. These serious epidemics usually strike every 10 to 40 years.

Influenza B, which causes smaller epidemics that are limited to specific localities. These types of epidemics usually appear every three to 15 years. Because global travel is so much more accessible today, this type of influenza can quickly spread to other places.

Influenza C, which is rare and causes mild symptoms. It is fairly stable and unchanging.

Types A and B are continually changing, making it difficult to create a vaccine that protects against the flu from one year to the next.

After having a strain of influenza virus, a person's body develops antibodies to the virus that protect from getting that particular strain again. However, because the strains change so often, people tend to get the flu more than once.

People who are at greatest risk of complications from influenza include:

  • Infants and children with chronic health problems, including asthma. These children may be more likely to have a severe illness if they contract the flu. It is especially important that they receive the vaccine.
  • Adults 65 and older.
  • Pregnant women.
  • People who have medical conditions that put them at higher risk for influenza-related complications. This includes people with asthma, diabetes, heart disease and cancer.

Individuals who suspect that they have the flu should see their doctor as soon as possible, especially if they're at high risk. People with serious symptoms, such as difficulty breathing, severe sore throat, coughing that produces a lot of green or yellow fluid or feelings of being faint, should see a doctor quickly. Other serious signs include severe cough, high fever and sharp pain when breathing deeply.

Having a flu shot each year may help lower your risk of catching influenza. The vaccine can be given to anyone who is 6 months or older.


To diagnose flu, a doctor will take a medical history and perform a physical examination. Flu can normally be diagnosed on the basis of the symptoms, the season of the year and local public health conditions. Your doctor may perform additional testing as needed to test for the presence of the flu virus.


A doctor may recommend bed rest and drinking plenty of fluids. Within the first day or two of having flu symptoms, certain drugs that fight viruses may be helpful.

Antiviral medications are used to treat influenza A and B. These antiviral drugs make inactive an enzyme that the virus needs to grow and spread. Zanamivir (Relenza®) is an antiviral medication that is inhaled every 12 hours for five days, and oseltamivir (Tamiflu®) is an antiviral medication in pill or liquid form. These drugs can shorten the length of the flu by a day or two and may help prevent spreading it to other family members. An IV antiviral medication peramivir (Rapivab®) is also available. All antiviral medications require a prescription.

These steps may help flu sufferers feel better:

  • Get plenty of sleep.
  • Drink plenty of liquids.
  • Have some chicken soup, which can help break up congestion as well as providing nutrition and fluid.
  • Take an over-the-counter pain reliever, such as acetaminophen, aspirin or ibuprofen. These will not decrease the flu's duration, but they can help those infected feel better. Be aware of the side effects, which can include stomach pain, bleeding and ulcers. Acetaminophen may harm the liver if taken for a long time or in large doses. Do not give acetaminophen to children without a doctor's recommendation, because it may cause liver or kidney damage. Aspirin should not be given to anyone younger than 16 because it may cause Reye's syndrome, a rare but potentially fatal disease.

The following steps are the best protection against getting influenza:

  • The first and most important way to prevent the flu is to get the flu vaccine every year.
  • Stay away from others who are sick. If you become sick, stay at home to minimize the risk of spreading the virus to others.
  • Wash hands often, especially during the winter, when the flu is most common. An alcohol-based hand rub can be used if soap and water are not available.
  • Keep hands away from nose, eyes and mouth. Viruses are most likely to enter the body through these areas.
  • Stop smoking. It irritates the lining of the nose, sinuses and lungs, which may make the body susceptible to complications of the flu and could affect your child's health.

Adults receive the flu vaccine in a single injection. Children 6 months through 8 years who are getting their first flu shot will need two shots about a month apart. The vaccine requires one to two weeks to take effect.

The vaccine is made from a form of the virus that doesn't have the power to infect you with the disease. No one can get the flu from a flu shot. Individuals may be sore where the shot was given or have mildly achy muscles or a fever that starts six to 24 hours after getting the shot. These symptoms last only a day or two and are most likely in children who have never been exposed to the flu virus.

All people 6 months and older should get the flu shot, with rare exceptions. Those who have had a severe allergic reaction to the flu vaccine or components of the vaccine should not get vaccinated. If you've had a history of Guillain-Barré syndrome or have an allergy to eggs, you should talk with your doctor before getting the vaccine.