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WHAT IS THE FLU?

Written by WebMD Editorial Contributors
Medically Reviewed by Carmelita Swiner, MD on January 28, 2022


ARTICLES ON WHAT IS INFLUENZA?

 * What Is the Flu?
 * History of the Flu
 * Types of Flu
 * Where Does Flu Come From?
 * Flu FAQs
 * Flu Glossary

Worried about catching the flu? Want to learn some ways to prevent it? Then read
on to learn more about influenza -- what it is, how it's spread, and who's at
greatest risk for getting it. Knowledge is power when it comes to preventing
flu!


WHAT IS THE FLU?

Influenza, commonly known as the "flu," is an extremely contagious respiratory
illness caused by influenza A or B viruses. Flu appears most frequently in
winter and early spring. The flu virus attacks the body by spreading through the
upper and/or lower respiratory tract.


WHAT'S THE DIFFERENCE BETWEEN A COLD AND FLU?

The common cold and flu are both contagious viral infections of the respiratory
tract. Although the symptoms can be similar, flu is much worse. A cold may drag
you down a bit, but the flu can make you shudder at the very thought of getting
out of bed.

Congestion, sore throat, and sneezing are common with colds. Both cold and flu
may bring coughing (less common), headache, and chest discomfort. With the flu,
though, you are likely to run a high fever for several days and have body aches,
fatigue, and weakness. Symptoms of the flu also tend to come on abruptly.
Usually, complications from colds are relatively minor, but a case of flu can
lead to a life-threatening illness such as pneumonia.

More than 100 types of cold viruses are known, and new strains of flu evolve
every few years. Since both diseases are viral, antibiotics cannot conquer cold
or flu. Remember: Antibiotics only treat bacterial infections.

Four antiviral medications are available to treat flu. But there are no
medications that specifically defeat the common cold. Antibiotics may be helpful
only if there is a secondary bacterial infection.



For in-depth information, see WebMD's Flu Treatment.


HOW ARE STOMACH FLU AND INFLUENZA DIFFERENT?

"Stomach flu" is a popular term, but not a true medical diagnosis. It's not
uncommon to mistake gastroenteritis, which is what stomach flu is, for the viral
infection we commonly call the "flu." Gastroenteritis refers to inflammation of
the gastrointestinal tract (stomach and intestines). Viruses are the most common
cause of stomach flu. With gastroenteritis, you may have symptoms such as fever,
abdominal cramps, nausea, vomiting, and diarrhea.

For more about gastrointestinal flu, read WebMD's Stomach Flu or Influenza?








HOW IS FLU SPREAD?

The flu virus is spread from person to person through respiratory secretions and
typically sweeps through large groups of people who spend time in close contact,
such as in daycare facilities, classrooms, college dormitories, military
barracks, offices, and nursing homes.



Flu is spread when you inhale droplets in the air that contain the flu virus,
make direct contact with respiratory secretions through sharing drinks or
utensils, or handle items contaminated by an infected person. In the latter
case, the flu virus on your skin can infect you when you touch or rub your eyes,
nose, or mouth. That's why frequent and thorough handwashing is a key way to
limit the spread of influenza. Flu symptoms start to develop from one to four
days after infection with the virus.


WHO'S AT GREATEST RISK FOR FLU COMPLICATIONS?

While anyone can get flu, infants, the elderly, pregnant women, and people with
chronic ailments such as diabetes, heart disease, lung disease, and HIV are at
highest risk for flu complications. Despite advances in flu prevention and
treatment, the CDC estimates that deaths related to influenza range from 3,000
to 49,000 deaths in the United States each year.

Specific strains of flu can be prevented by a flu vaccine, either a flu shot or
nasal spray flu vaccine. In addition, antiviral medications are available to
prevent flu. These drugs may help reduce the severity and the duration of flu
and are best used within the first 48 hours of the appearance of flu symptoms.

For in-depth information, see WebMD's Flu Complications.


ARE THERE DIFFERENT TYPES OF FLU VIRUSES?

Researchers divide flu viruses into three general categories: types A, B, and C.
All three types can mutate, or change into new strains, and type A influenza
mutates often, yielding new strains of the virus every few years. This means
that you can never develop a permanent immunity to influenza. Even if you
develop antibodies against a flu virus one year, those antibodies are unlikely
to protect you against a new strain of the flu virus the next year.

Type A mutations are responsible for major flu epidemics every few years and for
the major pandemics that can occur, though rarely. Type B is less common and
generally results in milder cases of flu. However, major flu epidemics can occur
with type B every three to five years.

Type C causes infection but does not cause typical flu symptoms. Both influenza
A and B have been linked to the development of Reye's syndrome, a potentially
fatal complication that usually affects children and teens under age 18.
Widespread outbreaks of Reye's syndrome have occurred with influenza type B and
also with chickenpox, but other viruses have been implicated. The risk of Reye's
syndrome is increased when taking aspirin, so  anyone under age 18 should not
take aspirin if they have any viral symptoms or are recovering from the flu or
any other virus.

Most influenza viruses that infect humans seem to originate in parts of Asia,
where close contact between livestock and people creates a hospitable
environment for mutation and transmission of viruses. Swine, or pigs, can catch
both avian (meaning from birds, such as poultry) and human forms of a virus and
act as hosts for these different viral strains to meet and mutate into new
forms. The swine then transmit the new form of the virus to people in the same
way in which people infect each other -- by transmitting viruses through
droplets in the air that people breathe in.



For in-depth information, see WebMD's Types of Flu.


WHAT IS AVIAN OR BIRD FLU?

Bird flu, or avian influenza, is an infectious disease of birds caused by type A
strains of the influenza virus. Bird flu epidemics have occurred worldwide.

Bird flu is a leading contender to be the next pandemic flu bug because it has
caused an unprecedented epidemic in poultry and wild birds across Asia and
Eastern Europe. Still, no one knows for sure whether this will cause the next
human flu pandemic.

For in-depth information, see WebMD's Understanding Avian or Bird Flu.

SHOW SOURCES

SOURCES:

National Institute of Allergy and Infectious Diseases: "Flu (Influenza):
Symptoms."

CDC: "Key facts about seasonal Influenza (Flu)," "Estimating Seasonal
Influenza-Associated Deaths in the United States: CDC Study Confirms Variability
of Flu."

National Jewish Medical and Research Center: "Influenza and the Flu Vaccine."

American Lung Association: "Cold and Flu Guidelines: Influenza."

FDA/CFAN: "What consumers need to know about avian influenza."

American Lung Association: "Myths and Facts About the Flu and Colds."

World Health Organization: "Influenza (Seasonal)."

Mayo Clinic: "Reye's syndrome."

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NEXT IN WHAT IS INFLUENZA?

History of the Flu



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 * Got the Flu? 10 Tips to Ease Symptoms
 * Flu Symptoms Topics




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