Nearly everyone is familiar with the influenza virus, since five to 20 percent of the U.S. population gets sick with the flu every year, according to the Centers for Disease Control and Prevention (CDC). But you may be surprised to learn there are three types of influenza viruses that affect humans — types A, B and C — and these viruses have different characteristics.
Influenza A is a common flu virus categorized by a few different subtypes and strains — most notably the H1N1 and H3N2 viruses that cause infection in humans. It can spread quickly and lead to severe infections, so it's helpful to understand signs and symptoms of influenza A.
What Is Influenza A?
Influenza is a highly contagious respiratory illness caused by an airborne virus. While influenza A, B and C viruses all affect humans, the A virus is unique because it's also commonly found in birds, pigs and other animals.
This variety of hosts causes the already unstable type A flu virus to rapidly and unpredictably mutate — which can lead to infections the human immune system is not well equipped to fight. As a result, the influenza A virus is capable of causing flu pandemics, or a worldwide spread of this infectious disease.
Influenza A Symptoms
While flu infections can vary in severity, all influenza viruses produce similar symptoms. For instance, the influenza C virus tends to cause only mild respiratory illnesses, but influenza A and B, the viruses responsible for seasonal flu outbreaks, can lead to more severe symptoms — and can be fatal.
The type A flu virus is generally thought to produce more severe symptoms compared to influenza B, however a 2016 study published in Clinical Infectious Diseases of more than 26,000 hospitalized flu victims discovered both types had comparable hospitalization rates, symptom severity and death rates.
Some cases of the flu are mild and may be mistaken for the common cold. But typical influenza A symptoms, which range from mild to severe, include:
- Tiredness or weakness
- Generalized aches and pains
- Headache, which may include eye discomfort and sensitivity to light
- Nonproductive cough, which can progress to a raspy and productive cough
- Scratchy, sore throat
- Runny or stuffy nose
- Burning sensation below or behind the sternum
Gastrointestinal symptoms, such as nausea, vomiting or abdominal pain may also occur with the flu, but these are more common in children than adults. Flu symptoms can last awhile, as the characteristic cough, weakness, tiredness and sweating may persist for weeks after infection.
Flu complications can be moderate, such as sinus or ear infections, or may be more severe and cause death. Serious consequences of the flu include pneumonia, inflammation of the heart muscle, brain or muscle tissues, or multi-system organ failure.
While anyone can get sick and suffer serious complications of the flu, those at highest risk include children less than 2 years of age, pregnant women, people over the age of 65 and those with compromised immune systems or chronic health problems. The type A flu virus disproportionately affects older adults and individuals with chronic health conditions, while influenza B tends to affect children more than other age groups.
How the Flu Spreads
The flu is passed from person to person through the air. When an infected person coughs or sneezes, airborne droplets of the influenza virus can enter the mouth, nose or lungs of people nearby. A person might also become infected by touching a surface that has the virus on it, such as a door knob, and then touching his mouth or nose.
According to the CDC, adults with the flu are contagious one day before showing symptoms and up to five to seven days after becoming sick. Children and people with weakened immune systems may be contagious for a longer time period.
Symptoms usually appear within one to four days of the virus entering the body. Some people may be infected with the flu virus but have no symptoms, yet they may still pass the virus to others.
Influenza A Treatment
Contact your doctor if you have symptoms of the flu. Influenza A is treated the same as any other type of flu infection — managing symptoms with bed rest, fluids and medications as needed to relieve aches and fever.
Your doctor may also prescribe antiviral medications. When started within the first two days, these medications can reduce the duration of symptoms.
Prevention of Influenza Type A
The CDC recommends that everyone six months of age and older get the annual flu vaccine, unless contraindicated due to allergy or other health reason. The vaccine is especially important for high-risk groups, including young children, pregnant women and the elderly.
While the vaccine doesn't protect against all flu viruses, it does protect against those that research indicates will be most prevalent in that year's flu season. Annual flu vaccines typically include protection against two influenza A viruses, and one to two type B viruses.
In addition to getting the vaccine, try to avoid close contact with people who are sick. If you are sick with the flu, stay home and minimize contact with others until you are fever free — without the help of a fever-reducing medication — for at least 24 hours.
Cover your mouth and nose with a tissue when you cough or sneeze, and dispose of tissues properly. Wash your hands often with soap and water, and avoid the spread of germs by keeping your hands from touching your eyes, nose and mouth.
Reviewed by Kay Peck, MPH RD
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- Centers for Disease Control and Prevention: Types of Influenza Viruses
- BMJ: Influenza
- Merck Manual: Influenza
- Centers for Disease Control and Prevention: Pandemic Basics
- Infection Ecology and Epidemiology: Influenza A Viruses: An Ecology Review
- Journal of Infectious Diseases and Therapy: How Influenza A Virus Causes ‘Epidemics’ and ‘Pandemics’ Among the Populations?
- Clinical Infectious Diseases: Comparing Clinical Characteristics Between Hospitalized Adults With Laboratory-Confirmed Influenza A and B Virus Infection
- Centers for Disease Control and Prevention: Seasonal Flu