The H1N1 flu, sometimes called swine flu, is a type of influenza A virus.

During the 2009-10 flu season, a new H1N1 virus began causing illness in humans. It was often called swine flu and was a new combination of influenza viruses that infect pigs, birds and humans.

The World Health Organization (WHO) declared the H1N1 flu to be a pandemic in 2009. That year the virus caused an estimated 284,400 deaths worldwide. In August 2010, WHO declared the pandemic over. But the H1N1 flu strain from the pandemic became one of the strains that cause seasonal flu.

Most people with the flu get better on their own.

But flu and its complications can be deadly, especially for people at high risk. The seasonal flu vaccine can now help protect against the H1N1 flu and other seasonal flu viruses.

The symptoms of flu caused by H1N1, commonly called the swine flu, are similar to those of other flu viruses.

Symptoms usually start quickly and can include:

• Fever, but not always.
• Aching muscles.
• Chills and sweats.
• Cough.
• Sore throat.
• Runny or stuffy nose.
• Watery, red eyes.
• Eye pain.
• Body aches.
• Headache.
• Tiredness and weakness.
• Diarrhea.
• Feeling sick to the stomach, vomiting, but this is more common in children than adults.

Flu symptoms develop about 1 to 4 days after you're exposed to the virus.

When to see a doctor

If you're generally healthy and develop flu symptoms, most people may not need to see a health care provider. But some people are at higher risk of flu complications.

Call your care provider if you have flu symptoms and you're pregnant or have a chronic disease. Some examples are asthma, emphysema, diabetes or a heart condition.

If you have emergency symptoms of the flu, get medical care right away. For adults, emergency symptoms can include:

• Trouble breathing or shortness of breath.
• Chest pain.
• Signs of dehydration such as not urinating.
• Ongoing dizziness.
• Seizures.
• Worsening of existing medical conditions.
• Severe weakness or muscle pain.

Emergency symptoms in children can include:

• Trouble breathing.
• Pale, gray or blue-colored skin, lips or nail beds depending on skin color.
• Chest pain.
• Dehydration.
• Severe muscle pain.
• Seizures.
• Worsening of existing medical conditions.

Influenza viruses such as H1N1 infect the cells that line your nose, throat and lungs. The virus spreads through the air in droplets released when someone with the virus coughs, sneezes, breathes or talks. The virus enters your body when you breathe in contaminated droplets. It also can enter your body if you touch a contaminated surface and then touch your eyes, nose or mouth.

You can't catch swine flu from eating pork.

People with the virus are likely able to spread the virus from about a day before symptoms appear until about four days after they start. Children and people with weakened immune systems may be able to spread the virus for a slightly longer time.

Factors that may increase your risk of developing H1N1 or other influenza viruses or their complications include:

• Age. Influenza tends to have worse outcomes in children under age 2, and adults older than age 65.
• Living or working conditions. People who live or work in facilities with many other residents are more likely to get the flu. Some examples are nursing homes or military barracks. People who are staying in the hospital also are at higher risk.
• Weakened immune system. Cancer treatments, anti-rejection medications, long-term use of steroids, organ transplant, blood cancer or HIV/AIDS can weaken the immune system. This can make it easier to catch the flu and may increase the risk of developing complications.
• Chronic illnesses. Chronic conditions may increase the risk of influenza complications. Examples include asthma and other lung diseases, diabetes, heart disease, and nervous system diseases. Other examples are metabolic disorders, problems with an airway and kidney, liver or blood disease.
• Race. American Indians or Alaska Native people may have a higher risk of influenza complications.
• Aspirin use under age 19. People on long-term aspirin therapy and who are younger than 19 years of age are at risk of Reye syndrome if infected with influenza.
• Pregnancy. Pregnant people are more likely to develop influenza complications, especially in the second and third trimesters. This risk continues up to two weeks after the baby is born.
• Obesity. People with a body mass index (BMI) of 40 or higher have a higher risk of flu complications.

Influenza complications include:

• Worsening of chronic conditions, such as heart disease and asthma.
• Pneumonia.
• Neurological symptoms, ranging from confusion to seizures.
• Respiratory failure.
• Bronchitis.
• Muscle tenderness.
• Bacterial infections.

The Centers for Disease Control and Prevention (CDC) recommends annual flu vaccination for everyone age 6 months or older. The H1N1 virus is included in the seasonal flu vaccine.

The flu vaccine can lower your risk of getting the flu. It also can lower the risk of having serious illness from the flu and needing to stay in the hospital.

Each year's seasonal flu vaccine protects against the three or four influenza viruses. These are the viruses expected to be the most common during that year's flu season.

Flu vaccination is especially important because the flu and coronavirus disease 2019 (COVID-19) cause similar symptoms.

Both COVID-19 and the flu may be spreading at the same time. Vaccination is the best way to protect against both. Flu vaccination could lessen symptoms that might be confused with those caused by COVID-19.

Vaccination also helps lower the number of people with severe flu and complications. And that may lower the number of people needing to stay in the hospital.

The flu vaccine is available as an injection and as a nasal spray.

The nasal spray is approved for people between 2 and 49 years old. It isn't recommended for some groups, such as:

• Children younger than age 2.
• Adults age 50 and older.
• Pregnant people.
• Children between 2 and 17 years old who are taking aspirin or a salicylate-containing medication.
• People with weakened immune systems.
• Close contacts or caregivers of people with highly weakened immune systems. Examples are people receiving chemotherapy, or recent bone marrow or solid organ transplantation.
• Children 2 to 4 years old who have had asthma or wheezing in the past 12 months.

If you have an egg allergy you can still get a flu vaccine.

These measures also help prevent the flu and limit its spread:

• Wash your hands often. If available, use soap and water, washing for at least 20 seconds. Or use an alcohol-based hand sanitizer that has at least 60% alcohol.
• Cover your coughs and sneezes. Cough or sneeze into a tissue or your elbow. Then wash your hands.
• Avoid touching your face. Avoid touching your eyes, nose and mouth.
• Clean and disinfect surfaces. Regularly clean often-touched surfaces to prevent spread of infection from a surface with the virus on it to your body.
• Avoid contact with the virus. Try to avoid people who are sick or have symptoms of flu. And if you have symptoms, stay home if you can. When flu is spreading, consider keeping distance between yourself and others while indoors, especially in areas with poor air flow. If you're at high risk of complications from the flu consider avoiding swine barns at seasonal fairs and elsewhere.

Your health care provider may do a physical exam to look for symptoms of influenza, including H1N1 flu, often called swine flu. Or the provider may review your symptoms with you over the phone.

The provider may order a test that finds influenza viruses such as H1N1.

There are many tests used to diagnose influenza. But not everyone who has the flu needs to be tested. In most cases, knowing that someone has the flu doesn't change the treatment plan.

Care providers are more likely to use a test to diagnose flu if:

• You're already in the hospital.
• You're at high risk of complications from the flu.
• You live with someone who is at greater risk of flu complications.

Your care provider may use a test to find out whether a flu virus is the cause of your symptoms. Or the tests may be used to get more information to see if another condition is causing your symptoms, such as:

• Heart problems, such as heart failure or an infection of the heart muscle.
• Lung and breathing problems, such as asthma or pneumonia.
• Brain and nervous system problems, such as encephalopathy or encephalitis.
• Septic shock or organ failure.

A test called polymerase chain reaction (PCR) may be used to see if you have the flu. It is more sensitive than other types of tests and may be able to find the flu strain.

It is possible to have both flu and another virus such as COVID-19 at the same time.

Most people with flu, including H1N1 flu, need only symptom relief. Supportive care such as drinking liquids, taking pain relievers for fever and headache, and resting may be helpful.

If you have a chronic respiratory disease, your health care provider may prescribe medications to help relieve your symptoms.

Health care providers may prescribe antiviral drugs within the first day or two of symptoms. They can reduce the severity of symptoms and may lower the risk of complications.

The U.S. Food and Drug Administration has approved these four antiviral drugs to treat flu:

• Oseltamivir (Tamiflu).
• Zanamivir (Relenza).
• Peramivir (Rapivab).
• Baloxavir (Xofluza).

But flu viruses can develop resistance to these drugs. So health care providers reserve antivirals for certain groups. This includes people at high risk of complications and those who are in close contact with people who have a high risk of complications.

Using antivirals carefully makes development of resistance less likely and keeps supplies of these drugs for those who need them most.

If you develop any type of flu, stay home. Keep sick children home until the fever has been gone for 24 hours.

These measures may help ease your symptoms:

• Drink plenty of liquids. Choose water, juice and warm soups to prevent dehydration.
• Rest. Get more sleep to help your immune system fight infection.
• Consider pain relievers. Use a nonprescription pain reliever, such as acetaminophen (Tylenol, others) or ibuprofen (Advil, Motrin IB, others). Use caution when giving aspirin to children or teenagers. Children and teenagers recovering from flu-like symptoms should never take aspirin. This is due to the risk of Reye syndrome, a rare but potentially life-threatening condition.

Avoid being around other people until you're feeling better, unless you're getting medical care. If you do need to leave your home and get medical care, wear a face mask. Wash your hands often.