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H1N1 (swine) influenza


The H1N1 virus (swine flu) is a new flu virus strain that has caused a worldwide pandemic in humans from June 2009 to August 2010.

The Centers for Disease Control and Prevention now call the virus 2009 H1N1.

Alternative Names:

Swine flu

Causes, incidence, and risk factors:

Earlier forms of the H1N1 virus were found in pigs. Over time, the virus changed (mutated) and can now infect humans. Because H1N1 is a new virus in humans, your immune system cannot fight the virus very well. As a result, it has spread quickly around the world.

The largest number of H1N1 flu cases have occurred in people ages 5 - 24. Few cases have been reported in people older than age 64.

The H1N1 flu virus can spread from person to person when:

  • Someone with the flu coughs or sneezes into air that others breathe in.
  • Someone touches a door knob, desk, computer, or counter with the H1N1 germs on it and then touches their mouth, eyes, or nose.
  • Someone touches mucus of a child or others while taking care of them when they are ill with the H1N1 flu virus

You CANNOT get H1N1 flu virus from eating pork or any other food, drinking water, swimming in pools, or using a hot tubs or saunas.


Symptoms of H1N1 flu infection in humans are similar to classic flu-like symptoms, which might include:

Signs and tests:

In general, most people do not need to be tested for it when they have symptoms.

Your doctor may test you for the H1N1 flu virus by swabbing the back of the inside of your nose if:

  • You are at high risk for flu complications.
  • Others at high risk of flu complications have been in close contact with you.
  • You are very sick.

Your doctor may:


Most people who get H1N1 flu will recover without needing medical care or special antiviral medications. Check with your health care provider about whether you should take antiviral medications to treat the H1N1 flu.

Doctors may prescribe antiviral drugs to treat people who become very sick with the flu or are at high risk for flu complications. The following people may be at high risk:

  • Children younger than 5 years old, especially those younger than age 2
  • Adults 65 years of age and older
  • People with:
    • Chronic lung (including asthma ) or heart conditions (except high blood pressure )
    • Kidney, liver, neurologic, and neuromuscular conditions
    • Blood disorders (including sickle cell disease)
    • Diabetes and other metabolic disorders
    • An immune system that does not work well, such as AIDS patients or cancer patients receiving chemotherapy

Other high risk people include:

  • Pregnant women
  • Anyone younger than age 19 receiving long-term aspirin therapy
  • Residents of nursing homes and other chronic-care facilities

People who may receive antiviral medications after coming into close contact with a person who is known to have, or probably is infected with the H1N1 virus, include:

  • Those at high risk for complications of influenza
  • Health care workers, public health workers, or first responders

Oseltamivir or zanamivir are the two drugs recommended for the treatment or prevention of infection with the H1N1, or swine, influenza virus.

People with H1N1 flu should also:

  • Get plenty of rest
  • Drink clear fluids (such as water, broth, sports drinks, and electrolyte beverages for infants)
  • Watch for emergency warning signs (see below)

Expectations (prognosis):

The outlook depends on the severity of the infection, age, and whether there are other medical problems.

Pregnant women and young people appear more likely to get the H1N1 virus and also to have bad outcomes when they become infected.

People age 65 or older have a lower risk than younger age groups.


Severe illness may occur along with:

Like seasonal flu, H1N1 flu may make other chronic medical problems worse.

Calling your health care provider:

Anyone who is pregnant, has young children, or has a health condition such as diabetes , heart disease , asthma , or emphysema should check with their doctor when they become ill.

If you are ill and have any of the following warning signs, seek emergency medical care.

In children, emergency signs include:

  • Fast breathing or trouble breathing
  • Bluish or gray skin color
  • Not drinking enough fluids
  • Severe or persistent vomiting
  • Not waking up or not interacting
  • Being so irritable that the child does not want to be held
  • Flu-like symptoms improve but then return with fever and a worse cough

In adults, emergency signs include:


The 2010 seasonal flue vaccine now protects against swine flu.

See also: Influenza vaccine


2009 H1N1 flu . Centers for Disease Control and Prevention. Accessed October 4, 2010

Seasonal Influenza (Flu): What you should know about flu antiviral drugs . Centers for Disease Control and Prevention. Accessed October 4, 2010

Committee on Infectious Diseases. Policy Statement: Recommendations for prevention and control of influenza in children, 2010-2011. Pediatrics. 2010 Aug 30.

Review Date: 10/3/2010
Reviewed By: David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.

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