What Causes the Flu?

“Most people are familiar with typical flu symptoms: that achey feeling, a sore throat, a fever, headache, and fatigue. What is the flu virus that causes these symptoms? And what’s the difference between the types of influenza?

Learn about the flu, how it spreads, and how you can avoid it.

What Causes the Flu?

Two types of influenza virus can cause the flu, influenza A and influenza B. Another flu virus, influenza C, can also infect humans. But this one usually only causes cold-like symptoms and isn’t linked to the surge in hospitalizations every flu season.

There are many different strains of Influenza A and B, with different genetic make-ups.

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What’s the Difference Between Influenza A and B?

Influenza A is the most common type of flu virus. It usually accounts for about 75% of flu illness. It mostly spreads earlier in the flu season, typically during the fall and early winter. Influenza B, which makes up about 25% of flu illness, tens to spreads later in the winter and into the spring.

Many experts say influenza A causes more severe symptoms. But a large U.S. study found the same mortality rates among hospitalized patients with influenza A and B.

Because influenza A is faster at mutating, it can get past immune defenses more often. Also, while influenza B only spreads in humans, influenza A circulates in animals too, including birds, chickens, and pigs.

This means influenza A can mutate in animals and then jump back into humans. When this happens, the virus is often more contagious and can cause more severe symptoms. That’s because the virus is quite different than the ones our immune systems have encountered in the past.

How Do You Catch the Flu?

Both influenza A and influenza B spread the same way. Many times, people catch the flu from someone who doesn’t yet have symptoms. The flu is contagious about one day before symptoms occur. People continue to be contagious in the first week of symptoms, but generally don’t spread the virus after day seven.

The flu spreads from droplets of liquid people produce when coughing, sneezing, and talking. Scientists increasingly think that aerosols — tiny particles produced when talking and even breathing — are also important in transmitting the disease. Unlike droplets, which fall to the ground, aerosols are light enough to float in the air for many minutes and even hours.

The flu virus spreads much better in poorly ventilated and indoor areas, compared to well-ventilated and outdoor environments. Moisture makes a difference too, with research showing the virus lives much longer in dry air compared to humid air.

You can also catch the flu by touching someone’s hands — or touching surfaces with virus on it — before touching your mouth. This is a less common route of transmission than breathing the virus in directly.

Do Influenza A and B Have Different Symptoms?

Influenza A and influenza B cause the same typical flu symptoms. These include:

  • Fever
  • Weakness
  • Cough
  • Body aches
  • Runny nose
  • Sore throat
  • Headaches

However, influenza B can also sometimes cause diarrhea and vomiting, especially in kids.

How Do You Treat the Flu?

In most cases, people can treat these symptoms at home, with rest and liquids, such as broth, juice, herbal tea, and water. Over-the-counter (OTC) medications, like acetaminophen and ibuprofen, can treat fever, body aches, and headaches. OTC cold medications can help with cough and a stuffy nose.

However, those at risk of getting serious complications from the flu, due to age or underlying illness, should call their health provider.

Doctors may recommend antiviral drugs for people at risk of getting very sick from the flu. Most of the antiviral drugs prescribed for flu work for both influenza A and influenza B. Some drugs only work for a specific type; doctors test patients first if using these drugs.

Antiviral drugs work best if patients take them within the first two days of symptoms. Some people who get the flu may need hospital care for dehydration or complications like pneumonia. People with pneumonia may have shortness of breath, chest pain, and confusion, on top of other flu symptoms.

How Do You Know Which Type of Flu Virus You Have?

Both types of flu cause similar symptoms and often respond to the same treatments at home and at the hospital. Unless your doctor wants to prescribe medication that only works against one subtype, you don’t need know what type you have.

However, scientists do track different flu subtypes and strains to see which strains are actively spreading.

This helps scientists create more effective vaccines for the next flu season. It also lets health providers know if a certain strain is causing more severe symptoms.

The CDC updates charts weekly to show what subtypes of influenza are currently spreading in the U.S.

How Can I Prevent the Flu?

You can prevent the flu by getting an annual flu vaccine. The flu vaccine includes two influenza A strains and two influenza B strains. This means the vaccine provides protection regardless of whether you get influenza A or B.

Scientists predict which flu strains to target with a vaccine, based on the strains that spread in the southern hemisphere. The vaccine is not always a close enough match to stop you from becoming infected with the flu. But the flu vaccine will still reduce the risk you will get severely sick if you do get the flu.

The Centers for Disease Control and Prevention recommends everyone in the U.S. who is eligible for the flu vaccine get vaccinated in September or October. This primes the immune system to fight the flu in time for flu season.

You can also prevent the spread of the flu by covering your mouth and nose when you sneeze or cough. If you are at risk of serious complications of the flu, you may wish to wear a mask in crowded indoor spaces.

Remember to wash your hands often. This has the benefit of protecting you not only from the flu, but also from a number of other viruses and bacteria.

Editor's Note: This article was originally published on , and was last reviewed on .

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Dr. Mayumbo Nyirenda et al. Estimating the Lineage Dynamics of Human Influenza B Viruses. PLoS One. Link

Emily Shiffer. What Is Influenza B—And What Are The Symptoms? Women's Health. Link

Dr. Timothy Uyeki et al. Influenza. Lancet. Link

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