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What to know about influenza A

The influenza A virus is highly contagious. It can spread through tiny droplets of bodily fluid from coughing, sneezing, or talking. Someone may even catch the flu by touching their mouth or nose after coming into contact with a surface or object that has the virus on it.

What is influenza A?
a woman with Influenza a who is coughing.
A cough is common symptom of influenza A.

The four types of influenza are A, B, C, and D. Types A and B are widespreadTrusted Source during most winters in the United States. Type C is milder than types A or B and does not spread as easily. Type D influenza mostly affects cattle and not humans.

Scientists divide influenza A into two further subtypes based on the proteins that live on the surface of the virus. These proteins, hemagglutinin (H) and neuraminidase (N), help the virus to attach to cells in the body, causing the infection.

There are 18 different H subtypes in influenza A that run from influenza H1 to H18. There are 11 N subtypes, ranging from N1 to N11. Each subtype also has different strains that influence the virus further.


The symptoms of flu will usually come on suddenly. They include:

In more severe cases, some people experience vomiting and diarrhea. These symptoms tend to be more common in children than in adults.

In most cases, the body's immune system will fight the virus off itself. But some people will experience complications. These are more common in older adults and those with conditions that affect their immune systems. Taking immunosuppressant medications can also increase the risk of complications.

Examples of possible complications from the flu include:

Flu can also worsen existing health conditions, such as asthma or congestive heart failure. Complications can become very severe, even life-threatening.


Most cases of flu are mild and will go away on their own within 2 weeksTrusted Source.

It is crucial to stay at home for the first few days after getting the infection to avoid spreading the virus to others. During this time, drink plenty of fluids and rest as much as possible.

A range of over-the-counter medications can help ease symptoms. For example, decongestants help to clear a blocked nose, and cough suppressants can ease throat pain from coughing. These medications do not treat the virus itself and cannot shorten illness duration.

People at risk of complications may need antiviral drugs to fight the virus. Examples of these medications include oseltamivir (Tamiflu) or zanamivir (Relenza). They can reduce recovery time by a couple of days.

A doctor will prescribe antiviral drugs to most people with the flu over the age of 65, or with a weakened immune system.

It is essential to speak to a doctor if the following symptoms occur:

  • body temperature over 101°F
  • coughs that produce a green or yellow substance
  • breathlessness while resting
  • faintness or dizziness
  • uncontrollable shaking or shuddering
hands getting washed under a tap
A person can regularly wash their hands to help prevent the spread of flu.

To help prevent the flu from spreading, people should wash their hands regularly, especially during the winter months.

Several vaccines are effective at preventing the flu. Vaccines work by preparing the immune system to make antibodies that fight off the virus before it can take effect.

Vaccines are specific for each strain of the virus. The vaccines available will vary depending on what strains of the flu doctors predict will spread at different times of the year.

Most available vaccines are quadrivalent. This means they protect people from two subtypes of influenza A and two subtypes of influenza B.

The CDC recommend that anyone 6 months of age and older has the vaccine unless they have any contraindicationsTrusted Source. They especially recommend it for people who have a higher chance of developing complications, including:

  • adults over the age of 65
  • pregnant women
  • young children
  • people with asthma
  • people with heart disease
  • people who have had a stroke
  • people with diabetes
  • people with HIV or AIDS
  • people with cancer
  • children with a neurological condition

People who are in close contact with anyone who is at risk of complications should also make sure they have had the vaccination. This includes doctors, nurses, or anyone who works in a medical environment.

It is necessary to have the flu vaccination yearly.


Influenza A will usually clear up on its own within 2 weeks. Stay at home for the first few days, and get plenty of rest and water. Over-the-counter medications can help to ease symptoms but do not shorten the illness duration.

The flu is highly contagious. It is best to avoid close contact with other people as much as possible when recovering from the flu. Avoid all contact with anyone who is at risk of complications from the flu.

Vaccinations are available to prevent the flu. These vaccinations are essential for people at risk of complications. Developing the flu in these cases can have very severe consequences, which can become life-threatening.



Medically reviewed by Alana Biggers, MD, MPH  New — Written by Amanda Barrell

"what to know about influenza A," October 15, 2019

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