What Is Influenza A?

<p>Oscar Wong / Getty Images</p>

Oscar Wong / Getty Images

Medically reviewed by Jenny Sweigard, MD

Influenza A is the most common type of flu virus that makes you sick during the colder months. This type of flu is highly contagious. In fact, you can catch the illness by inhaling or ingesting the germs of someone who has the flu anytime they cough, sneeze, or talk near you.

Every year, anywhere from 3% to 11% of people develop influenza A. Children and infants under the age of 18 and adults over the age of 65 are most at risk of developing seasonal flu symptoms—although, anyone can experience the flu. If you contract influenza A, you'll likely experience symptoms that affect your nose, throat, and lungs—such as fever, cough, fatigue, chills, and body aches.

Fortunately, influenza A often goes away on its own with proper rest and at-home remedies. But, medical treatments are available to help soothe your symptoms. Implementing prevention strategies can also protect you from the illness or reduce the risk of experiencing more serious complications.

Types of Influenza Viruses

There are four primary types of influenza viruses, which include:

  • Influenza A: The most common and contagious type of flu

  • Influenza B: A less common type of virus that also causes the seasonal flu

  • Influenza C: A rarer type of flu that causes mild, cold-like symptoms, often affecting young children under the age of two

  • Influenza D: Doesn't produce symptoms in humans, but does cause illness in animals such as cattle

Symptoms of Influenza A

Everyone experiences flu symptoms a little differently. Symptoms can also last anywhere from a week to 10 days. But, your symptoms may linger for a longer period of time or be more severe if you are younger than 18 or older than 65 and live with an underlying health condition or autoimmune disorder.

Common Symptoms

Influenza A primarily affects your respiratory system—which includes your nose, throat, and lungs. Symptoms of influenza A can come on suddenly and may include:

Severe Symptoms

In severe cases, influenza A can cause an additional set of symptoms. These symptoms are rare, but may sometimes become a medical emergency:


Influenza A can happen when the virus infects the tissues in your upper respiratory tract (your sinuses, mouth, and throat) or your lower respiratory tract (your lungs). The virus can replicate within these tissues, which can cause symptoms to develop.

If you have influenza A and are experiencing symptoms, this type of flu is highly contagious. You can transmit the virus to someone else by coughing, sneezing, or speaking near someone else. Similarly, you can contract the virus if someone who is sick passes their germs to you when they cough, sneeze, or talk.

This type of flu virus often mutates (or, changes) rapidly every flu season. Even if you've had the flu before, your immune system might not be able to immediately fight off newer forms of the virus. As a result, you may experience symptoms for a few days as your immune system fights the infection.

Risk Factors

While anyone can develop influenza A, certain people are at greater risk for having symptoms and complications. This is the case for:


Healthcare providers mainly rely on a physical exam and medical history to diagnose you for influenza A. During flu season, these assessments are enough for a proper diagnosis.

But, if your symptoms last for a longer period of time, your provider may order laboratory tests to rule out other conditions, such as viral infections, Dengue fever, or COVID-19, among other illnesses. There are several diagnostic tests that providers use, such as:

  • Rapid antigen test: Detects the presence of a virus after collecting a sample of your tissue from your nose or throat, which can show results in as quickly as 15 minutes

  • Rapid molecular assay for nucleic acid detection: Delivers results within 15 to 30 minutes after getting a sample of your tissue from a swab test to identify viruses

  • Polymerase chain reaction (PCR) test: Identifies influenza viruses from samples of your tissue, which can take anywhere from 45 minutes to several hours to offer results

  • Immunofluorescence: Looks for influenza and other viruses using a specialized microscope to examine specimens taken from a sample of tissue from your sinuses or throat

  • Viral culture: Uses a throat or nasal swab to check for signs of influenza—but is no longer commonly used as results can take several days to come back

  • Chest X-ray: Takes images of the lungs to help your provider rule out bacterial infections of the lungs or pneumonia


Getting enough rest and drinking more fluids can help your body fight off influenza A without the need for additional medical treatment. But if your symptoms aren't going away or feel severe, antiviral medications (which attack viruses and prevent transmitting the illness to others) can help.

For best results, healthcare providers recommend starting this treatment within two days of developing symptoms. These medications can often reduce the duration of the illness and the severity of your symptoms. Your healthcare provider may prescribe the following drugs to ease your symptoms:

  • Tamiflu (oseltamivir)

  • Relenza Diskhaler (zanamivir)

  • Rapivab (peramivir)

How to Prevent Influenza A

It's easy to catch influenza A during flu season because the illness is so contagious. But, implementing prevention strategies can protect you from developing the flu or reduce your risk of experiencing complications if you do get sick. Some prevention methods include:

  • Getting the annual flu shot

  • Limiting contact with people who have the flu

  • Covering your mouth when you sneeze or cough

  • Wearing a mask to prevent contracting the infection

  • Washing your hands regularly and thoroughly

  • Avoiding touching your face as germs can easily spread from your hands to your eyes, nose, or mouth

  • Disinfecting and cleaning surfaces or objects that may have come into contact with someone who has the flu


While most cases of influenza A resolve within a couple of weeks, sometimes the flu can be more severe and lead to complications in some people. Seek care from your healthcare provider if you or a loved one suspect symptoms of any of the following complications:

A Quick Review

Influenza A is the most common type of flu virus that causes seasonal epidemics of the flu during the colder months. This illness is highly contagious and causes symptoms like fever, cough, nasal congestion, and fatigue.

Anyone can develop influenza A, but people under the age of 18 or over the age of 65 and those who have underlying health conditions are most at risk of experiencing severe symptoms and complications.

While many cases of influenza A resolve on their own with proper rest, antiviral medications can help reduce the duration of illness and prevent infections. Getting vaccinated, washing your hands, and limiting contact with someone who has the flu can reduce your own risk of developing the illness.

Frequently Asked Questions

Is flu A or B worse?

Flu outbreaks are caused by two strains of influenza virus: influenza A and B. Generally, influenza A tends to be more common and severe than influenza B.

How long should you stay home with influenza A?

If you have influenza A, it’s important to take steps not to spread the disease. The current guidance from the Centers for Disease Control and Prevention (CDC) is to stay home for at least 24 hours after your fever goes away. The exception to this rule is leaving for medical care or immediate basic necessity.

Will influenza A go away on its own?

In most cases, influenza A resolves on its own without the need for medical treatment within one to two weeks after you start experiencing symptoms. But, if you or your child have an underlying condition, it's common to experience more serious symptoms. That said, consider reaching out to your provider to see which treatments they recommend.

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