The Flu, NIAID Fact Sheet: NIAID


Article title: The Flu, NIAID Fact Sheet: NIAID
Conditions: Flu, Reye's syndrome
Source: NIAID
July 2001


Influenza, or the flu, is a respiratory infection caused by a variety of flu viruses. The most familiar aspect of the flu is the way it can "knock you off your feet" as it sweeps through entire communities.

The flu differs in several ways from the common cold, a respiratory infection also caused by viruses. For example, people with colds rarely get fevers or headaches or suffer from the extreme exhaustion that flu viruses cause.

The U.S. Centers for Disease Control and Prevention (CDC) estimates that 35 to 50 million Americans come down with the flu during each flu season, which typically lasts from November to March. Children are two to three times more likely than adults to get sick with the flu, and children frequently spread the virus to others. Although most people recover from the illness, CDC estimates that in the United States more than 100,000 people are hospitalized and more than 20,000 people die from the flu and its complications every year.

When and Where Do People Usually Get the Flu?

Flu outbreaks usually begin suddenly and occur mainly in the late fall and winter. The disease spreads through communities creating an epidemic. During the epidemic, the number of cases peaks in about three weeks and subsides after another three or four weeks. Half of the population of a community may be affected. Because schools are an excellent place for flu viruses to attack and spread, families with school-age children have more infections than other families, with an average of one-third of the family members infected each year.

Is the Flu an Important Disease?

Besides the rapid start of the outbreaks and the large numbers of people affected, the flu is an important disease because it can cause serious complications. Most people who get the flu get better within a week (although they may have a lingering cough and tire easily for a while longer). For elderly people, newborn babies, and people with certain chronic illnesses, however, the flu and its complications can be life-threatening.

How is the Flu Transmitted?

You can get the flu if someone around you who has the flu coughs or sneezes. You can get the flu simply by touching a surface like a telephone or door knob that has been contaminated by a touch from someone who has the flu. The viruses can pass through the air and can enter your body through your nose or mouth. If you've touched a contaminated surface, they can pass from your hand to your nose or mouth.

You are at greatest risk of getting infected in highly populated areas, such as in crowded living conditions and in schools.

What are Flu Symptoms?

If you get infected by the flu virus, you will usually feel symptoms one to four days later. You can spread the flu to others before your symptoms start and for another three to four days after your symptoms appear. The symptoms start very quickly and are
  • Headache
  • Chills
  • Dry cough
  • Body aches
  • Fever
  • Stuffy nose
  • Sore throat
Typically, the fever begins to decline on the second or third day of the illness. The flu almost never causes symptoms in the stomach and intestines. The illness that some people often call "stomach flu" is not influenza.

How Does a Doctor Diagnose the Flu?

Usually, doctors or other health care workers diagnose the flu on the basis of whether flu is epidemic in the community and whether the patient's complaints fit the current pattern of symptoms. Doctors rarely use laboratory tests to identify the virus during an epidemic. Health officials, however, monitor certain U.S. health clinics and do laboratory tests to determine which type of flu virus is responsible for the epidemic.

How Can I Keep from Getting the Flu?

Flu Vaccine

The main way to keep from getting the flu is to get a yearly flu vaccine. You can get the vaccine at your doctor's office or a local clinic, and in many communities at workplaces, supermarkets, and drugstores. You must get the vaccine every year because it changes.

Scientists make a different vaccine every year because the strains of flu viruses change from year to year. Nine to 10 months before the flu season begins, they prepare a new vaccine made from inactivated (killed) flu viruses. Because the viruses are killed, they cannot cause infections. The vaccine preparation is based on the strains of the flu viruses that are in circulation at the time. It includes those A and B viruses (see section below on types of flu viruses) expected to circulate the following winter.

Sometimes, an unpredicted new strain may appear after the vaccine has been made and distributed to doctors and clinics. Because of this, even if you do get the flu vaccine, you still may get infected. If you do get infected, however, the disease usually is milder because the vaccine still will give you some protection.

Your immune system takes time to respond to the flu vaccine. Therefore, you should get vaccinated six to eight weeks before flu season begins to prevent getting infected or reduce the severity of flu if you do get it. The vaccine itself cannot cause the flu, but you could become exposed to the virus by someone else and get infected soon after you are vaccinated.

Are there possible side effects from the flu vaccine?

You should be aware that the flu vaccine can cause side effects. The most common side effect in children and adults is soreness at the site of the vaccination. Other side effects, especially in children who previously have not been exposed to the flu virus, include fever, tiredness, and sore muscles. These side effects may begin 6 to 12 hours after vaccination and may last for up to two days.

Viruses for producing the vaccine are grown in chicken eggs and then killed with a chemical so that they can no longer cause an infection. The flu vaccine may contain some egg protein, which can cause an allergic reaction. Therefore, if you are allergic to eggs or have ever had a serious allergic reaction to the flu vaccine, CDC recommends that you consult with your doctor before getting vaccinated.

Who should get the flu vaccine?

If you are in any of the following groups or live in a household with someone who is, CDC recommends that you get the flu vaccine.
  • You are 50 years of age or older.
  • You have chronic diseases of your heart, lungs, or kidneys.
  • You have diabetes.
  • Your immune system does not function properly.
  • You have a severe form of anemia.
  • You will be more than three months pregnant during the flu season.
  • You live in a nursing home or other chronic-care housing facility.
The vaccine can be administered to children as young as six months old. Children should get the flu vaccine if they are taking long-term aspirin treatment as they may be at risk of developing Reye's syndrome following a flu infection (see section on complications in children). They should also get the flu vaccine if they live in a household with someone in the above groups.

Health care workers and volunteers should get the flu vaccine if they work with patients in any of the above groups.

Medicine for Prevention

Although the flu vaccine is the best way to prevent getting the flu, three antiviral medicines also are available by prescription that will help prevent flu infection:
  • Tamiflu® (oseltamivir)
  • Flumadine® (rimantadine)
  • Symmetrel® (amantadine)
The Food and Drug Administration (FDA) has approved Tamiflu® for use in adults and adolescents 13 years and older. Rimantadine and amantadine have been approved for use by adults and children who are 1 year of age and older.

Rimantadine and amantadine have unpleasant side effects. Your doctor can help you decide which medicine is best for you.
  • These medicines help prevent the flu if you take them for at least two weeks during the outbreak of flu in your community.
  • You may use these medicines if you are in close contact with family members or others who have the flu.
  • You may use them if you are in close contact with people who have been vaccinated but whom you want to give added protection from getting the flu.
  • You may use either medicine immediately following flu vaccination during a flu epidemic to protect you during the two- to four-week period before antibodies (proteins from your immune system that protect you from the flu virus) develop or when a flu epidemic is caused by virus strains other than those covered by the vaccine.
You should discuss the flu vaccine and the medicines with your doctor before the flu season begins.

What is the Treatment for the Flu?

Many people treat their flu infections by simply
  • Resting in bed
  • Drinking plenty of fluids
  • Taking over-the-counter medicine such as aspirin or acetaminophen (Tylenol®, for example)
You should not give aspirin to children and adolescents who have the flu.

You should not take antibiotics to treat the flu because they do not work on viruses. Antibiotics only work against some infections caused by bacteria.

Medicine for Treatment

If you do get the flu and want to take medicine to treat it, your doctor may prescribe one of four available antiviral medicines:
  • Tamiflu® (oseltamivir) helps adults 18 years and older and Relenza® (zanamivir) helps adults and children 7 years and older who have an uncomplicated flu infection and who have had symptoms for no more than two days. FDA recently approved Tamiflu® for use in children 1 year of age and older who have had symptoms for no more than two days. Both treat influenza type A and type B infections.
  • Flumadine® (rimantadine) helps adults who have influenza type A virus infections. It has no effect on influenza type B virus infections.
  • Symmetrel® (amantadine) can be taken by adults and children who are 1 year of age and older to prevent and treat type A or type B influenza virus infections. Amantadine, however, is more likely to cause side effects such as lightheadedness and inability to sleep more often than is rimantadine.
To work well, you must take these medicines within 48 hours after the flu begins. They reduce the length or time fever and other symptoms last and allow you to return to your daily routine quicker.

What are Possible Complications from the Flu?

You can have flu complications if you get a bacterial infection, which causes pneumonia in your weakened lungs. Pneumonia also can be caused by the flu virus itself.

Symptoms of complications will usually appear after you start feeling better. After a brief period of improvement, you may suddenly get
  • High fever
  • Shaking chills
  • Chest pain with each breath
  • Coughing that produces thick, yellow-greenish-colored mucus
Pneumonia can be a very serious and sometimes life-threatening condition. If you have any of these symptoms, you should contact your doctor immediately so that you can get the appropriate treatment.

Are There Other Flu Complications that Only Affect Children?

Reye's syndrome, a condition that affects the nerves, sometimes develops in children and adolescents who are recovering from the flu. Reye's syndrome begins with nausea and vomiting, but the progressive mental changes (such as confusion or delirium) cause the greatest concern.

The syndrome often begins in young people after they take aspirin to get rid of fever or pain. Although very few children develop Reye's syndrome, you should consult a doctor before giving aspirin or products that contain aspirin to children. Acetaminophen does not seem to be associated with Reye's syndrome.

Other complications of the flu that affect children are
  • Convulsions caused by fever
  • Croup
  • Ear infections, such as otitis media
Newborn babies recently out of intensive care units are particularly vulnerable to suffering from flu complications.

Are There Different Types of Flu Viruses?

The first flu virus was identified in the 1930's. Since then, scientists have classified flu viruses into types A, B, and C.

Type A is the most common and usually causes the most serious epidemics. Type B outbreaks also can cause epidemics, but the disease it produces generally is milder than that caused by type A. Type C viruses, on the other hand, never have been connected with a large epidemic.

Flu Pandemics in the 20th Century

If a flu virus emerges that is either new or that has not circulated in many years, and if it is able to spread easily from person to person, it could quickly travel around the world and cause serious illness and death for millions of people. This is called a flu pandemic.

The 1918 Spanish flu pandemic is the catastrophe against which all modern pandemics are measured. More than 20 million people were killed worldwide; 500,000 died in the United States alone. This virus was especially quick to kill. So far, the world has not seen a virus that severe again.

In 1957 and 1968, the Asian flu and Hong Kong flu, respectively, invaded the United States. Although hundreds of thousands of people in the United States died, the death toll for each pandemic was not as high as that for the Spanish flu .

In 1976, the United States experienced a swine flu scare. When a new flu virus was first identified at Fort Dix, New Jersey, it was labeled the "killer flu," and health experts were afraid that it would infect people around the world. In fact, swine flu never left the Fort Dix area. Research on the virus later showed that if it had spread, it would probably have been much less deadly than the Spanish flu.

In 1997, another "near miss" pandemic occurred when 18 people in Hong Kong became ill from a new flu virus. Six of the infected people subsequently died. Usually, flu viruses move first from chickens to pigs, and then from pigs to humans. This virus was different because it moved directly from chickens to people. The avian flu never became a pandemic, however, because it didn't easily spread from person to person. In addition, public health authorities ordered the slaughter of all live chickens in Hong Kong.

What Research is Going On?

Although flu epidemics pop up in the fall and winter seasons in communities throughout the world every year, including the United States, there has not been a pandemic since 1968. Scientists are worried that a new flu virus will emerge in the 21st century and cause a severe pandemic again. For this reason, research institutions and health departments around the world are cooperating to track flu outbreaks in humans and animals, and to determine what types and strains of flu viruses are the causes.

To prevent another flu pandemic and reduce the numbers of flu epidemics, the National Institute of Allergy and Infectious Diseases (NIAID) supports research to find out how influenza viruses work, and to develop better vaccines to prevent and treat influenza virus infections.

For more information on NIAID influenza research, see "Focus on the Flu" on our Web site at

Where Can I Get More Information About the Flu?

  • National Institute on Aging Information Center

  • Centers for Disease Control and Prevention (CDC)
    National Immunization Information Program

  • Food and Drug Administration (FDA)
    Rockville, MD 20857
    1-888-INFO-FDA (1-888-463-6332)

NIAID is a component of the National Institutes of Health (NIH). NIAID supports basic and applied research to prevent, diagnose, and treat infectious and immune-mediated illnesses, including HIV/AIDS and other sexually transmitted diseases, tuberculosis, malaria, autoimmune disorders, asthma and allergies.

Prepared by:
Office of Communications and Public Liaison
National Institute of Allergy and Infectious Diseases
National Institutes of Health
Bethesda, MD 20892

Public Health Service
U.S. Department of Health and Human Services

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