H1N1 is a strain of influenza, or the flu. There are several different types of the flu — A, B, C, and D.

Influenza A and B cause seasonal epidemics throughout the cold months of the year. This time frame is often referred to as “flu season.”

Influenza A viruses are further classified into subtypes based on two proteins that are found on the surface of the virus:

  • hemagglutinin (H)
  • neuraminidase (N)

This is how you get names like H1N1 or H3N2.

Some people hear “H1N1” and instantly think of the swine flu that circulated in 2009. But H1N1 flu strains have circulated during flu season for a long time.

In 2009, an H1N1 strain, which came to be known as swine flu, popped up that was very different from other H1N1 strains. You might also see it referred to as the H1N1 pandemic (H1N1pdm09) virus.

Although the pandemic has been over for years now, the H1N1pdm09 virus continues to circulate as a seasonal influenza strain. It’s now included as one of the viruses that the seasonal flu vaccine protects against. Keep in mind that the flu vaccine isn’t 100 percent effective.

Read on to learn more about this type of flu, including its symptoms in adults, children, and babies.

Although the initial symptoms of the flu are similar to those of the common cold, symptoms often come on suddenly rather than gradually.

The symptoms of H1N1pdm09 flu are similar to those other types of flu and can include:

  • fever, which may not occur in all people
  • a runny or congested nose
  • sore throat
  • cough
  • headache
  • body aches and pains
  • chills
  • fatigue
  • loss of appetite
  • nausea or vomiting
  • diarrhea

Flu symptoms aren’t as easy to read in children and babies, largely because it’s harder for them to communicate what they’re feeling.

Look for the following symptoms if you suspect a child may have the H1N1pdm09 virus:

  • difficulty breathing
  • fussiness or irritability
  • issues waking up
  • not drinking sufficient fluids
  • confusion
  • rash that appears with a fever

Unlike bacterial infections, viral infections, including the flu, don’t respond to antibiotics. In most cases, you’ll just want to lay low, get plenty of rest, and drink as much fluid as you can.

However, some people have a higher risk of developing complications from an H1N1pdm09 infection and should see a healthcare provider if they have symptoms of the flu.

These groups include:

  • children under 5 years of age
  • adults ages 65 and older
  • pregnant people
  • people with a weakened immune system due to medication or an underlying disease
  • people living with with a chronic condition, such as asthma, diabetes, lung disease, and heart disease

If you or a loved one are at risk of complications, you may be prescribed antiviral medication, such as oseltamivir (Tamiflu). Antiviral medications may help to reduce symptom severity. But they work best when started a day or two after symptoms first appear, so try to get an appointment sooner rather than later.

Flu symptoms can sometimes be serious, even in those who aren’t in a high-risk group.

Seem immediate treatment if you or someone else experiences:

  • shortness of breath
  • breathing problems
  • pain or pressure in the chest or abdomen
  • sudden dizziness
  • confusion
  • severe or ongoing vomiting
  • flu symptoms that get better but return with a worse cough and fever

Additional symptoms in children and babies also warrant immediate medical attention:

  • rapid breathing
  • blue-tinted skin
  • irritability to the point of not wanting to be held
  • not drinking fluids
  • trouble waking up

If you or your child come down with the H1N1pdm09 virus but don’t have severe symptoms, get ready to spend at least a few days at home.

Ease symptoms and support the recovery process by:

  • getting plenty of rest
  • drinking fluids, including water, warm broth, or juice, as much as possible
  • taking over-the-counter fever reducers, such as ibuprofen (Advil, Motrin) or acetaminophen (Tylenol)
  • dressing in layers that are easy to add or remove if you’re having chills
Aspirin and children don’t mix

While ibuprofen and acetaminophen can provide temporary symptom relief, avoid giving children aspirin. This can lead to the development of a serious condition called Reye’s syndrome.

H1N1pdm09 is a flu virus that emerged in 2009, spreading quickly and causing a pandemic. The virus now circulates seasonally and is one of the types of flu that the seasonal flu vaccine can protect against.

The symptoms of H1N1pdm09 flu typically go away in about a week, but you may continue to feel fatigued for a couple of weeks afterward.

To avoid spreading the virus to others, try to stay home for at least 24 hours after your fever is gone.

If you or your child has a high risk for complications from the flu, see a healthcare provider as soon as possible.