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Flu Fact Sheet

Flu Fact Sheet

February 27, 2005 -- Influenza, or flu, is an acute respiratory infection caused by a variety of influenza viruses. The most familiar aspect of flu is the way it can "knock you off your feet" as it sweeps through entire communities, usually during the winter. Flu differs in several ways from the common cold, a respiratory infection also caused by viruses.


Outbreaks of flu usually begin abruptly. As the disease spreads through communities, the number of cases peaks in about 3 weeks and subsides after another 3 or 4 weeks. Twenty to fifty percent of a population may be affected, with the highest incidence in 5- to 14-year-olds. Schools are an excellent place for transmission of flu viruses, so that families with school-age children have a higher rate of infection than other families, with an average of one-third of the family members infected each year.

Besides the rapid onset of the outbreaks and the large numbers of people affected, flu is important because of the seriousness of the complications that can develop. Most people who contract the disease recover within a week (although they may tire easily for awhile). However, for elderly people, newborn babies, and people with certain chronic illnesses, flu and its complications can be life-threatening.


Viruses that cause flu spread primarily from person to person, especially by coughing and sneezing (via airborne droplets of respiratory fluids). Flu viruses can enter the body through the mucous membranes of the eyes, nose, or mouth. After a person has been infected with the virus, symptoms usually appear within 2 to 4 days. The infection is considered contagious for another 3 to 4 days after symptoms appear.

The greatest risk of infection is in highly populated areas, where people live in crowded conditions, and in schools. Isolating people with flu symptoms is not an effective means of disease control because flu can be spread by someone whose symptoms are not yet apparent.


Flu is usually signaled by headache, chills, and dry cough, which are followed rapidly by body aches and fever. Typically, the fever starts declining on the second or third day of the illness. It is then that the upper respiratory symptoms become noticeable__nasal congestion and sore throat. Flu almost never causes gastrointestinal symptoms; the illness that people often call "stomach flu" is not influenza.

Usually, doctors diagnose 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 testing to identify the virus. Health officials monitor certain U.S. health clinics and do tests to determine which type of flu virus is responsible for the epidemic.


Once a person has the flu, treatment usually consists of resting in bed, drinking plenty of fluids, and taking medication such as aspirin or paracetamol to relieve fever and discomfort. Children with flu should not take aspirin (see Complications). Antibiotics are not effective against flu viruses.

The drug rimantadine can be used to treat influenza type A virus infections in adults. It has no effect on influenza type B infections. When taken within 48 hours after the onset of illness, it reduces the duration of fever and other symptoms and allows flu sufferers to return to their daily routines more quickly.

Rimantadine is a derivative of the drug amantadine, which also can prevent and treat flu infection. Amantadine, however, is more likely to cause side effects such as lightheadedness and inability to sleep more often than is rimantadine.


Flu complications (which can either accompany or follow the illness) generally result from bacterial infections in the lower respiratory tract. The ensuing pneumonia usually is caused by pneumococcal bacteria, but infections with staphylococci, streptococci, and Haemophilus influenzae type b can occur.

Symptoms of complications usually appear after the flu patient starts feeling better. This brief period of improvement is followed by the sudden onset of high fever, shaking chills, chest pain with each breath, and coughing that produces thick, yellow-greenish-colored sputum. Although most people with pneumonia recover after treatment with antibiotics, some pneumonia-causing organisms are resistant to these drugs.

A neurologic disease known as Reye's syndrome sometimes develops in a small number of children and adolescents who are recovering from flu. Reye's syndrome usually is signaled by the onset of nausea and vomiting, but the progressive mental changes (such as confusion or delirium) cause the greatest concern. The syndrome is associated with the use of aspirin, which often is used in medications for relieving the pain or fever of flu. Although fewer than 3 children per 100,000 with flu develop Reye's syndrome, one should consult a physician before administering aspirin or aspirin-containing products to children. Use of paracetamol is not associated with Reye's syndrome.

Other complications of flu that affect children are fever-related convulsions, croup, and ear infections. Newborns recently out of intensive care units are particularly vulnerable.

Influenza 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 prevalent and is associated with the most serious epidemics. Type B outbreaks also can reach epidemic levels, 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.

Immune Responses to Flu Viruses

A flu virus looks like a ball studded with spikes. The spikes consist of molecules of two proteins (called antigens), hemagglutinin and neuraminidase, on the surface of the virus. When the virus attaches to a cell in a healthy person, these surface proteins stimulate the person's immune cells to produce antibodies, which are the proteins that fight invading microbes.

Each strain of virus has different surface antigens, and the immune system produces antibodies specific to each antigen. The antigens of an attacking virus also leave an imprint on the memory of the immune cells so that they can respond immediately to a second exposure of the same antigens and prevent a second infection.

Mutations and New Strains

Unlike antigens of other viruses, the surface antigens of flu viruses change periodically. These changes circumvent antibodies and complicate vaccine development.

The hemagglutinin molecules of flu viruses are highly unstable and often mutate during replication. These periodic changes result in new strains of viruses with altered surface antigens. Persons with antibodies stimulated either by previous infection or vaccination are not protected from infection with new strains of flu virus.

Slight changes in flu virus antigens are referred to as antigenic drifts. Every few years, at unpredictable intervals, a major change, or antigenic shift, occurs. Shifts have been observed only in influenza A viruses.

Pandemics are the result of antigenic shifts and are associated with severe illness and significant mortality on a global scale. Within this century, at least five pandemics and numerous epidemics (regional outbreaks involving fewer people) have occurred. For example, antigenic shifts were evident in the 1957 outbreak known as the Asian flu, which affected huge numbers of people, and in the 1968 outbreak of the virulent Hong Kong strain.

Although the viruses causing these two outbreaks were both type A, the hemagglutinin surface antigen in the 1968 Hong Kong strain was very different from that of the 1957 Asian strain. The Hong Kong outbreak was severe because the antibodies people had developed to protect them against the Asian strain were ineffective against the new strain.

The Russian flu pandemic in the winter of 1977-8 was caused by a strain of virus identical to the one that caused an epidemic in 1950. This virus had somehow been preserved in its original form for 27 years. The Russian pandemic primarily affected people under 25 years of age, indicating that people who had been exposed to the virus in 1950 had developed and maintained immunity. This phenomenon led scientists to recognize that the natural immunity resulting from influenza infection can last more than two decades.


Each year, scientists formulate a new vaccine made from inactivated (killed) influenza viruses. The preparation is based on the strains in circulation at the time, yet includes those A and B viruses expected to circulate the following winter. Sometimes, an unpredicted new strain may appear after the vaccine has been manufactured and distributed, resulting in infection even among those who received flu vaccine. Usually, however, the disease is milder because the vaccine will provide some protection.

Since the immune system takes time to respond to vaccination, the inactivated vaccine should be given 6 to 8 weeks before flu season begins in order to stimulate enough antibodies to prevent infection or reduce the severity of the illness. The vaccine itself cannot cause flu, but someone could become exposed and infected soon after vaccination, before antibodies develop.

The vaccine, however, may cause side effects, especially in children who previously have not been exposed to the flu virus. The most common side effect in children and adults is soreness at the site of the vaccination. Others include fever, tiredness and sore muscles that may begin 6 to 12 hours after vaccination and may last for up to 2 days.

Viruses for vaccine production are grown in chicken eggs and then inactivated with a chemical so that they are no longer infectious. People who are allergic to eggs should not receive flu vaccine since some egg protein may be present in the vaccine.

The drugs rimantadine and amantadine, which were discussed in the section on Treatment, also can be used to prevent flu in children aged 1 year and older and in healthy adults, if taken for about 6 weeks during the flu season. They may also be used by family members or close contacts of influenza A patients and by elderly nursing home residents who have been vaccinated but may need added protection. In addition, either drug may be used immediately following vaccination during a flu epidemic to provide protection during the 2- to 4-week period before antibodies develop or when a flu epidemic is caused by virus strains other than those covered by the vaccine.

Vaccination Guidelines

People in the following categories should ask their physicians about receiving vaccine each year to protect against flu.

High-Risk Groups:

  • All individuals aged 65 years or older.
  • People with chronic cardiovascular, pulmonary or metabolic disorders (including diabetes).
  • Those with renal dysfunction, anemia, immunosuppression, or asthma.
  • Residents of nursing homes and other chronic-care facilities.
  • Children receiving long-term aspirin therapy who may be at risk of developing Reye's syndrome following influenza infection.
  • Children 6 months or older with respiratory disorders.

Health-Care Workers:

  • Those who provide in-home care to high-risk patients.
  • Medical-care personnel with extensive patient contact.

In addition, physicians should administer vaccine to any individuals who want to reduce their chances of acquiring influenza.

NIAID, a component of the National Institutes of Health, supports research on AIDS, tuberculosis and other infectious diseases as well as allergies and immunology.

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
December 1997

Date First Published: September 6, 1999

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