The H1N1 flu, commonly known as swine flu, is primarily caused by the H1N1 strain of the flu (influenza) virus. H1N1 is a type of influenza A virus, and H1N1 is one of several flu virus strains that can cause the seasonal flu. Symptoms of the H1N1 flu are the same as those of the seasonal flu.

In the spring of 2009, scientists recognized a particular strain of flu virus known as H1N1. This virus is a combination of viruses from pigs, birds and humans that causes disease in humans. During the 2009-10 flu season, H1N1 caused the respiratory infection in humans that was commonly referred to as swine flu. Because so many people around the world got sick, in 2009 the World Health Organization (WHO) declared the H1N1 flu to be a pandemic. In August 2010, WHO declared the pandemic over. After the pandemic was over, the H1N1 flu virus became one of the strains that cause seasonal flu.

The flu vaccine can now help protect against the H1N1 flu (swine flu). The H1N1 flu virus strain is included in the seasonal flu vaccine, including the vaccine for 2020-21.

The signs and symptoms of flu caused by the H1N1 virus are similar to those of infections caused by other flu strains and can include:

• Fever, but not always
• Chills
• Cough
• Sore throat
• Runny or stuffy nose
• Watery, red eyes
• Body aches
• Headache
• Fatigue
• Diarrhea
• Nausea and vomiting

Flu symptoms develop about one to three days after you're exposed to the virus.

When to see a doctor

It's not necessary to see a doctor if you're generally healthy and develop flu signs and symptoms, such as fever, cough and body aches. Call your doctor, however, if you have flu symptoms and you're pregnant or you have a chronic disease, such as asthma, emphysema, diabetes or a heart condition, because you have a higher risk of flu complications.

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

• Difficulty breathing or shortness of breath
• Chest pain
• Ongoing dizziness
• Seizures
• Worsening of existing medical conditions
• Severe weakness or muscle pain

Emergency signs and symptoms in children can include:

• Difficulty breathing
• Blue lips
• 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 enters your body when you inhale contaminated droplets or transfer live virus from a contaminated surface to your eyes, nose or mouth.

You can't catch swine flu from eating pork.

If you live in or travel to an area where many people are infected with the H1N1 virus, you may be exposed to the virus.

Influenza complications include:

• Worsening of chronic conditions, such as heart disease and asthma
• Pneumonia
• Neurological signs and symptoms, ranging from confusion to seizures
• Respiratory failure

The Centers for Disease Control and Prevention (CDC) recommends annual flu vaccination for everyone age 6 months or older. Each year's seasonal flu vaccine protects against the three or four influenza viruses that are expected to be the most common during that year's flu season. The flu vaccine can reduce your risk of the flu and its severity and lower the risk of having serious illness from the flu and needing to stay in the hospital.

Flu vaccination is especially important in the 2020-21 flu season because the flu and coronavirus disease 2019 (COVID-19) cause similar symptoms. Flu vaccination could reduce symptoms that might be confused with those caused by COVID-19. Preventing the flu and reducing the severity of flu illness and hospitalizations could also lessen 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 use in healthy people ages 2 through 49 years old. The nasal spray isn't recommended for some groups, such as pregnant women, children between 2 and 4 years old with asthma or wheezing, and people who have compromised immune systems.

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

• Wash your hands thoroughly and frequently. Use soap and water, or if they're unavailable, use an alcohol-based hand sanitizer.
• 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 surfaces. Regularly clean often-touched surfaces to prevent spread of infection from a surface with the virus on it to your body.
• Avoid contact. Stay away from crowds if possible. Avoid anyone who is sick. If you're at high risk of complications from the flu — for example, you're younger than 5 or you're 65 or older, you're pregnant, or you have a chronic medical condition such as asthma — consider avoiding swine barns at seasonal fairs and elsewhere.

During the COVID-19 pandemic, both COVID-19 and the flu may be spreading at the same time. Your local health department and the CDC may suggest other precautions to reduce your risk of COVID-19 or the flu. For example, you may need to practice social distancing (physical distancing) and stay at least 6 feet (2 meters) from others outside your household. You may also need to wear a cloth face mask when around people outside your household.

Your doctor will conduct a physical exam, look for signs and symptoms of influenza, including H1N1 flu (swine flu), and possibly order a test that detects influenza viruses such as H1N1.

There are several tests used to diagnose influenza, but not everyone who has the flu needs to be tested. Your doctor may diagnose you with influenza based on your signs and symptoms. In most cases, knowing that someone has the flu doesn't change the treatment plan. Doctors 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 doctor may also use a test to determine whether a flu virus is the cause of your symptoms, or if you have or are showing signs of another problem besides the flu, 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

In some cases, your doctor may suggest that you be tested for influenza. He or she may use various tests to diagnose influenza. Polymerase chain reaction (PCR) testing is becoming more common in many hospitals and labs. This test may be done while you're in your doctor's office or in the hospital. PCR testing is more sensitive than other tests and may be able to identify the influenza strain.

During the COVID-19 pandemic, it's possible to have a test to diagnose both influenza and COVID-19. It's possible to have both COVID-19 and influenza at the same time.

Most people with flu, including H1N1 flu (swine flu), require 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 doctor may prescribe additional medications to help relieve your symptoms.

Antiviral drugs are sometimes prescribed within the first day or two of symptoms. They can reduce the severity of symptoms and possibly the risk of complications. The U.S. Food and Drug Administration has approved these four drugs:

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

But flu viruses can develop resistance to these drugs.

To make development of resistance less likely and maintain supplies of these drugs for those who need them most, doctors reserve antivirals for people at high risk of complications and those who are in close contact with people who have high risk of complications.

People at higher risk of flu complications include people who:

• Are in a hospital, nursing home or other long-term care facility.
• Are younger than 5 years of age, particularly children younger than 2 years.
• Are 65 years old or older.
• Are pregnant or within two weeks of delivery, including women who have had pregnancy loss.
• Are younger than 19 years of age and are receiving long-term aspirin therapy. Using aspirin during a viral illness increases the risk of developing Reye's syndrome, a rare but potentially life-threatening condition, in these individuals.
• Have a body mass index above 40, which is defined as morbid obesity.
• Have certain chronic medical conditions, such as asthma, emphysema, heart disease, diabetes, neuromuscular disease, or kidney, liver or blood disease.
• Are immunosuppressed due to certain medications or HIV.
• Are of American Indian or Alaska Native heritage.

If you develop any type of flu, 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 an over-the-counter pain reliever, such as acetaminophen (Tylenol, others) or ibuprofen (Advil, Motrin IB, others).

  Use caution when giving aspirin to children or teenagers. Though aspirin is approved for use in children older than age 3, children and teenagers recovering from chickenpox or flu-like symptoms should never take aspirin. This is because aspirin has been linked to Reye's syndrome, a rare but potentially life-threatening condition, in such children.

If you have the flu, you can give it to others. Stay home for at least 24 hours after your fever is gone.