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Influenza, or 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
The U.S. Centers for Disease Control and Prevention (CDC) estimates
that 10 to 20 percent of 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 about 36,000 people die from the flu and its
complications every year.
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 3 weeks and subsides after another 3 or 4
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.
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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.
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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 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.
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If you get infected by the flu virus, you will usually feel symptoms 1 to 4
days later. You can spread the flu to others before your symptoms start and
for another 3 to 4 days after your symptoms appear. The symptoms start very
quickly and may include
- Dry cough
- Body aches
- 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
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Usually, health care providers diagnose the flu on the basis of whether it is
epidemic in the community and whether the patient's complaints fit the current
pattern of symptoms. Health care providers 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.
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The main way to keep from getting flu is to get a yearly flu vaccine.
You can get the vaccine at your health care provider'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.
Until recently, you could get the flu vaccine only as an injection
(shot). In 2003, however, the Food and Drug Administration (FDA) approved
a nasal spray flu vaccine called FluMist, which you can get from your
health care provider. FDA approved it for use in healthy people aged 5 to
You should not use FluMist if
- You have certain lung conditions, including asthma, or heart
- You have metabolic disorders such as diabetes or renal dysfunction
- You have an immunodeficiency diseases or are on immunosuppressive
- You have had Guillain-Barré syndrome
- You are pregnant
- You have a history of allergy or hypersensitivity, including
anaphylaxis, to any of the parts of FluMist or to eggs
Children or teenagers who regularly take aspirin or products containing
aspirin also should not take FluMist.
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
Sometimes, an unpredicted new strain may appear after the vaccine has
been made and distributed to health care providers 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 6 to 8 weeks before flu season begins in
November to prevent getting infected or reduce the severity of flu if you
do get it. Because the flu season usually lasts until March, however, it's
not too late to get it after the season has begun. 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
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 2 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 health care provider 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 3 months pregnant during the flu season
- You live in a nursing home or other chronic-care housing facility
In 2004, CDC added the recommendation that children 6 months to 23
months of age get the flu vaccine every year. CDC also recommends that you
get a flu shot if you are in close contact with children in this age
Children and teenagers (aged 6 months to 18 years) 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 providers 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)
Tamiflu is for use in adults and teenagers 13 years and older.
Rimantadine and amantadine may be used by adults and children who are 1
year of age and older.
Rimantadine and amantadine have unpleasant side effects. Your health
care provider can help you decide which medicine is best for you.
These medicines help prevent the flu if you take them for at least 2
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
You may use either medicine immediately following flu vaccination
during a flu epidemic to protect you during the 2- to 4-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 medicines with your health care
provider before the flu season begins.
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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)
Do not give aspirin to children and adolescents who
have the flu.
Do 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
health care provider may prescribe one of four available antiviral
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 also has approved Tamiflu
for use in children 1 year of age and older who have had symptoms for no
more than 2 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
Symmetrel (amantadine) may be taken by adults and
children who are 1 year of age and older to prevent and treat type A
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.
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You can have flu complications if you get a bacterial infection, which can cause
pneumonia in your weakened lungs. Pneumonia also can be caused by the flu virus
Complications usually appear after you start feeling better. After a
brief period of improvement, you may suddenly get symptoms.
- 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
health care provider immediately so that you can get the appropriate
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Reye's syndrome, a condition that affects the nerves, sometimes develops in
children and teenagers 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 health care provider 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
- Ear infections, such as otitis media
Newborn babies recently out of intensive care units are particularly
vulnerable to suffering from flu complications.
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The first flu virus was identified in the 1930s. 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.
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If a flu virus emerges that is either new or 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
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.
In 1999, two children in Hong Kong were infected with a flu virus that
usually infects birds. They were the first confirmed human infections by
this virus, and both children recovered. Although other infections from
this virus were reported from China, there have been no cases since April
In 2003, there have been three avian flu outbreaks.
- In February, three people in Hong Kong were infected with an avian
influenza virus related to the 1997 virus; two died.
- In March, an influenza outbreak in poultry workers in the
Netherlands infected about 80 workers; one died.
- In November, one child in Hong Kong was infected but recovered.
The U.S. 2003 flu season began early, and as of December 6, 47 states
reported cases of human-to-human flu transmission to CDC. Twenty-four
states reported widespread infections. Several children have died in this
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Although flu epidemics pop up in the fall and winter seasons in communities
throughout the world every year, there has not been a pandemic since 1968. Scientists
are worried that a new flu virus will emerge in this 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
National Institute of Allergy
and Infectious Diseases
National Institute on Aging
National Library of Medicine
Centers for Disease Control
and Prevention National Immunization Information Program
Food and Drug Administration
American Lung Association
National Coalition for Adult Immunization
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