H1N1 Influenza (Swine Flu)

What Is H1N1 Flu?

H1N1 flu, sometimes called swine flu, is an infection caused by a new kind of influenza virus that spreads from one person to another. In 2009, the U.S. Centers for Disease Control and Prevention reported an outbreak of this new strain of flu in Mexico, the United States, and other countries. The virus that causes this type of flu is the H1N1 virus.

When you have H1N1 flu, you may feel tired and achy and have a sore throat, a fever, and a cough. Most of the time, the illness is not serious. But in some cases H1N1 flu can be severe and can lead to pneumonia, serious lung problems, and death.


H1N1 flu is caused by the H1N1 influenza virus. At first, experts thought that the virus was spread from pigs to people. But they later found that H1N1 flu is caused by a new virus. After a person is infected, he or she may spread the flu to other people by coughing or sneezing.

Drops from a cough or sneeze from an infected person can move through the air. Breathing them in can make you sick. You can also get infected by touching something with the flu virus on it, such as a desk or counter, and then touching your mouth or nose.

You can't get H1N1 flu from eating pork.


The symptoms of H1N1 flu are a lot like the regular seasonal flu. They can include:

  • Fever or shaking chills.
  • Cough.
  • Sore throat.
  • Body aches.
  • Headache.
  • Extreme tiredness (fatigue).

Some people also have vomiting and diarrhea. Most of the time, the illness is not serious. But in some cases, H1N1 flu can be severe and can lead to pneumonia, serious lung problems, and death.

Some people are more likely to have serious problems from H1N1 flu, including:

  • Pregnant women. The developing baby also is at risk for problems if the mother gets H1N1 flu.
  • People with long-term (chronic) health problems such as asthma or diabetes.
  • People with a weak immune system.
  • People who have serious heart or kidney disease or some diseases that affect nerves and muscles.
  • People younger than 25 years old.

Incubation and contagious periods

The time between getting infected with H1N1 virus and feeling sick (called the incubation period) is usually about 2 days but can be from 1 to 7 days.1 You can infect others starting 1 day before your symptoms start. And you may infect others up to 12 days after you get sick, although most people are infectious for about 6 days.1

When to Call a Doctor

For your child

Call 911 or other emergency services if:

  • Your child has severe trouble breathing. Signs may include the chest sinking in, using belly muscles to breathe, or nostrils flaring while your child is struggling to breathe.

Call your doctor now or seek immediate medical care if your child:

  • Has a fever with a stiff neck, a severe headache, or a rash.
  • Is confused, does not know where he or she is, or is extremely sleepy or hard to wake up.
  • Has trouble breathing, breathes very fast, or coughs all the time.
  • Has signs of needing more fluids. These signs include sunken eyes with few tears, dry mouth with little or no spit, and little or no urine for 8 or more hours.

For you

Call 911 or other emergency services if:

  • You have severe trouble breathing.

Call your doctor now or seek immediate medical care if you:

  • Have increased trouble breathing.
  • Have a fever with a stiff neck or a severe headache.
  • Feel extremely sleepy or confused.

If you think you have the symptoms of H1N1 flu or are worried that you have been exposed to the virus, call your doctor or go to the Centers for Disease Control and Prevention Web site (www.cdc.gov/H1N1flu) for more information on what to do.

Exams and Tests

If your doctor thinks you have H1N1 flu, he or she will do a physical exam and ask you questions about your symptoms and past health.

Your doctor may also take a nasal swab to test for the virus. If this test is needed, it is usually done within the first 4 or 5 days of illness. But it may not always detect the H1N1 flu virus even if you have H1N1 flu.


If you get sick:

  • Talk to your doctor.
    • If you are not at high risk for problems from the flu and do not have severe illness, your doctor may recommend that you take care of yourself at home.
    • If you are at high risk or are very sick, your doctor may prescribe an antiviral medicine such as oseltamivir (Tamiflu) or zanamivir (Relenza) to help you feel better faster and prevent problems caused by the flu. The sooner you start taking the medicine, the better it works.
  • Get extra rest.
  • Drink plenty of fluids to replace those lost from fever.
  • Take acetaminophen (such as Tylenol), ibuprofen (such as Advil or Motrin), or naproxen (such as Aleve) to relieve fever, headache, and body aches. Do not give aspirin to anyone younger than 20. You may also sponge your body with lukewarm water to reduce fever. Do not use cold water or ice.

If you are infected with H1N1 flu, you can infect others starting 1 day before your symptoms start. It's also possible that you may infect others up to 12 days after you get sick, although most people are infectious for about 6 days.1 To avoid spreading the flu to others during this time:

  • Stay home from school or work until you are feeling better and your fever has been gone for at least 24 hours. The fever needs to have gone away on its own without the help of medicine.
  • Try to avoid being around other people. If you have to be around people (including those you live with), wear a mask over your nose and mouth if you can.
  • Cover your nose and mouth with a tissue when you cough or sneeze. Throw the tissue in the trash after you use it. If you don't have a tissue, cover your mouth when you cough or sneeze. Use the bend in your arm, rather than using your hands.
  • Wash your hands often, especially if you are coughing or sneezing.


A vaccine can help prevent you from getting H1N1 flu. People who most need the vaccine are:2

  • Pregnant women.
  • People who live with or care for children younger than 6 months of age.
  • Health care workers who take care of sick people.
  • People ages 6 months to 24 years.
  • People 25 to 64 years old who have long-term (chronic) health problems (such as asthma or diabetes) or who have a weak immune system.

There are two types of the H1N1 vaccine: a shot(What is a PDF document?) , and a nasal spray(What is a PDF document?) that you breathe in through your nose. The nasal spray vaccine cannot be given to pregnant women and people with certain health problems, because it contains a weakened but live form of the virus. But these groups can get the H1N1 shot. Check with your doctor or local health department to find out which type of vaccine is best for you.

Deciding About Getting the H1N1 Influenza (Swine Flu) Vaccine

Here are some other things you can do to keep from getting sick:

  • Avoid close contact with others who are sick.
  • Wash your hands often, using soap and water. Alcohol-based hand cleaners also work well.
  • Avoid touching your eyes, nose, and mouth. Germs spread this way.
  • Try not to touch surfaces that may be contaminated with the virus. Some viruses and bacteria can live for 2 hours or longer on surfaces such as doorknobs, cafeteria tables, and desks.
  • If you are at high risk for serious problems from the flu, consider wearing a mask over your nose and mouth when you are in confined or crowded spaces, such as on an airplane. When possible, avoid being in a crowd.
  • Try to stay in good general health. Get plenty of rest, eat healthy foods, and drink lots of fluids.

Latest Information About H1N1 Flu

These organizations are studying and keeping track of H1N1 flu, including what is being done to prevent its spread. Their Web sites have the most up-to-date information about H1N1 flu:

  • U.S. Government. You can find information at www.pandemicflu.gov.
  • U.S. Centers for Disease Control and Prevention (CDC). You can find information at www.cdc.gov/H1N1flu.
  • World Health Organization (WHO). You can find information at www.who.int/csr/disease/swineflu/en.

Other Places To Get Help


Centers for Disease Control and Prevention (CDC): Novel H1N1 Flu
1600 Clifton Road
Atlanta, GA 30333
Phone: 1-800-CDC-INFO (1-800-232-4636)
TDD: 1-888-232-6348
E-mail: cdcinfo@cdc.gov
Web Address: www.cdc.gov/h1n1flu

The H1N1 flu site of the CDC provides the latest information on H1N1 flu (initially called swine flu), including:

  • Steps you can take to prevent H1N1 flu, and information about the H1N1 flu vaccine.
  • What to do if you or a family member gets sick.
  • National and international surveillance reports.
  • Facts and figures.
  • Background information about the beginnings of H1N1 flu.



  1. Cao B, et al. (2009). Clinical features of the initial cases of 2009 pandemic influenza A (H1N1) virus infection in China. New England Journal of Medicine, 361(26): 2507–2515.
  2. Centers for Disease Control and Prevention (2009). Use of influenza A (H1N1) 2009 monovalent vaccine: Recommendations of the Advisory Committee on Immunization Practices (ACIP), 2009. MMWR, 58(RR-10): 1–8. Also available online: http://www.cdc.gov/mmwr/pdf/rr/rr5810.pdf.

Other Works Consulted

  • Centers for Disease Control and Prevention (2009). Update on influenza A (H1N1) 2009 monovalent vaccines. MMWR, 58(39): 1100–1101. Also available online: http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5839a3.htm?s_cid=mm5839a3_e.
  • Novel Swine-Origin Influenza A (H1N1) Virus Investigation Team (2009). Emergence of a novel swine-origin influenza A (H1N1) virus in humans. New England Journal of Medicine, 360(25): 2605–2615.
  • Schnitzler SU, Schnitzler P (2009). An update on swine-origin influenza virus A/H1N1: A review. Virus Genes, 39(3): 279–292.


Author Maria Essig
Editor Katy E. Magee, MA
Associate Editor Tracy Landauer
Primary Medical Reviewer Adam Husney, MD - Family Medicine
Specialist Medical Reviewer Christine Hahn, MD - Epidemiology
Last Updated November 10, 2009

Last Updated: November 10, 2009

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