flu is an illness caused by a virus. Like a cold, it attacks the nose, throat,
and lungs. The flu can sometimes lead to other problems like pneumonia, ear and
sinus problems, dehydration, and worsening of asthma. For most of us, the flu
will go away in one-two weeks.
(or common) flu is a respiratory illness that can be transmitted person to
person. Most people have some immunity, and a vaccine is available.
Avian (or bird) flu is
caused by influenza viruses that occur naturally among wild birds. The H5N1
variant is deadly to domestic fowl and can be transmitted from birds to humans.
There is no human immunity and no vaccine is available.
Pandemic flu is virulent
human flu that causes a global outbreak, or pandemic, of serious illness.
Because there is little natural immunity, the disease can spread easily from
person to person.
Every year in the United States,
- 5% to 20% of the population
gets the flu;
- more than 200,000 people are
hospitalized from flu complications, and;
- about 36,000 people die from
Symptoms of flu include:
- fever (usually high)
- extreme tiredness
- dry cough
- sore throat
- runny or stuffy nose
- muscle aches
- Stomach symptoms,
such as nausea, vomiting, and diarrhea, also can occur but are
more common in children than adults
Complications of flu can include bacterial
pneumonia, ear infections, sinus infections, dehydration, and worsening of
chronic medical conditions, such as congestive heart failure, asthma, or
How Flu Spreads
Flu viruses spread mainly from person to
person through coughing or sneezing of people with influenza. Sometimes people
may become infected by touching something with flu viruses on it and then
touching their mouth or nose. Most healthy adults may be able to infect others
beginning 1 day before symptoms develop and up to 5 days after
becoming sick. That means that you may be able to pass on the flu to
someone else before you know you are sick, as well as while you are sick.
What We Know About the Flu Virus
A flu virus is roughly round, but
it can also be elongated or irregularly shaped. Inside are eight segments of
single-strand RNA containing the genetic instructions for making new copies of
the virus. Flu's most striking feature is a layer of spikes projecting from its
surface. There are two different types of spikes: one is the protein
hemagglutinin (HA), which allows the virus to "stick" to a cell and
initiate infection, the other is a protein called neuraminidase (NA), which
enables newly formed viruses to exit the host cell.
Virus A, B, C
Influenza viruses are classified
as type A, B, or C based upon their protein composition. Type A viruses are
found in many kinds of animals, including ducks, chickens, pigs, whales, and
also in humans. The type B virus widely circulates in humans. Type C has been
found in humans, pigs, and dogs and causes mild respiratory infections, but does
not spark epidemics.
Type A influenza is the most
frightening of the three. It is believed responsible for the global outbreaks of
1918, 1957 and 1968. Type A viruses are subdivided into groups based on two
surface proteins, HA and NA. Scientists have characterized 16 HA subtypes and 9
Naming Viral Strains
Type A subtypes are classified by
a naming system that includes the place the strain was first found, a lab
identification number, the year of discovery, and, in parentheses, the type of
HA and NA it possesses, for example, A/Hong Kong/156/97 (H5N1). If the virus
infects non-humans, the host species is included before the geographical site,
as in A/Chicken/Hong Kong/G9/97 (H9N2). There are no type B or C subtypes.
Where Influenza Comes From
In nature, the flu virus is found
in wild aquatic birds such as ducks and shore birds. It has persisted in these
birds for millions of years and does not typically harm them. But the frequently
mutating flu viruses can readily jump the species barrier from wild birds to
domesticated ducks and then to chickens. From there, the next stop in the
infectious chain is often pigs.
Pigs can be infected by both bird
(avian) influenza and the form of influenza that infects humans. In a setting
such as a farm where chickens, humans and pigs live in close proximity, pigs act
as an influenza virus mixing bowl. If a pig is infected with avian and human flu
simultaneously, the two types of virus may exchange genes. Such a "reassorted"
flu virus can sometimes spread from pigs to people.
Depending on the precise
assortment of bird-type flu proteins that make it into the human population, the
flu may be more or less severe.
In 1997, for the first time,
scientists found that bird influenza skipped the pig step and infected humans
directly. Alarmed health officials feared a worldwide epidemic (a pandemic).
But, fortunately, the virus could not pass between people and thus did not spark
an epidemic. Scientists speculate that chickens may now also have the receptor
used by human-type viruses.
Drifting and Shifting
Influenza virus is one of the
most changeable of viruses. These genetic changes may be small and continuous or
large and abrupt.
Small, continuous changes happen
in type A and type B influenza as the virus makes copies of itself. The process
is called antigenic drift. The drifting is frequent enough to make the new
strain of virus often unrecognizable to the human immune system. For this
reason, a new flu vaccine must be produced each year to combat that year's
Type A influenza also undergoes
infrequent and sudden changes, called antigenic shift. Antigenic shift occurs
when two different flu strains infect the same cell and exchange genetic
material. The novel assortment of HA or NA proteins in a shifted virus creates a
new influenza A subtype. Because people have little or no immunity to such a new
subtype, their appearance tends to coincide with very severe flu epidemics or
Habits for Good
These steps may help prevent the
spread of respiratory illnesses such as the flu:
Cover your nose and mouth
with a tissue when you cough or sneeze - throw the tissue away after
you use it.
- Wash your hands often
with soap and water, especially after you cough or sneeze. If you
are not near water, use an alcohol-based hand cleaner.
- Avoid close contact
with people who are sick. When you are sick, keep your distance
from others to protect them from getting sick too.
- If you get the flu,
stay home from work, school, and social gatherings. In this way you
will help prevent others from catching your illness.
- Try not to touch your
eyes, nose, or mouth. Germs often spread this way.
How can I catch the flu?
- The flu is easily passed from
person to person by coughing and sneezing.
- A person can also get the flu
by touching something with the flu virus on it and then touching their mouth
Who is most at risk for getting
Everyone is at risk for getting the flu,
but for some people the flu can cause serious illness. Those most at risk
- Older adults (over age 65)
- Pregnant women
- Very young children
- Adults and children (6 months and
older) who have heart or lung disease, including asthma.
- Adults and children (6 months and
older) who have diabetes, kidney or blood problems.
- People with HIV/AIDS, cancer or any
condition that make it harder to fight off disease
- Children and teens (6 months to 18
years) who take aspirin for a long time
- People who live in nursing homes and
other health care facilities
- Workers in hospitals or clinics who are
around lots of people who have the flu. These people should contact their
doctor or clinic if they have flu-like symptoms.
What are the signs of the flu?
- Headache and muscle ache
- Sore throat
- Runny or stuffy nose
- Throwing up or diarrhea (more common in
How well does the flu shot work?
- The shot prevents the flu in 70% to 90%
of young, healthy adults.
- The shot doesn't do as well at
preventing flu in older adults and people with certain medical problems. But
the shot does reduce the number of these people who die or need a hospital
stay because of the flu.
People who are allergic to eggs or who
have had a reaction to the flu shot or vaccine should not be given the flu
shot or nasal flu spray. Also, talk to your doctor if you have a history of
How well does nasal spray
- This new vaccine can lower your chances
of getting the flu. Children 5-8 years old need two doses at least 6 weeks
apart in their first year of getting the Flu Mist, and people 9-49 need one
- Flu Mist should not be given to people
with asthma or other lung diseases. Children under the age of 5 should not
get the spray.
Are there drugs to treat the
flu? Yes, there are a number of products that may help you feel better:
- Prescription medicines can lessen your
symptoms or the time you are sick with the flu. Your doctor or clinic will
help you decide whether these drugs are right for you.
- Over-the-counter medicines can help
with flu symptoms such as sore throat, stuffy nose, cough, fever and body
- Never give aspirin to children or
teens who might have the flu. It can cause serious problems or even
death. Call your doctor or clinic first.
- If you already take prescription
medicines, ask your doctor or pharmacist which flu medicines you can use
safely. For example, some over-the-counter flu products have medicines
to treat a stuffy nose (decongestants), which can raise your blood
pressure or even make your blood pressure medicines less effective.
- No medicine can take the place of
the flu vaccine
What should I do if I get the
- Get plenty of rest
- Drink lots of water or other liquids
like juice and soup
- Don't spread your germs! Cover your
mouth when you cough or sneeze, wash your hands often, and STAY HOME!
- Don't smoke or drink alcohol.
Many other diseases can feel like the
flu, but they need different treatments. Always go to your doctor or clinic if
you feel worse.
Credit: CDC, NIH, Food and Drug