There are four types of influenza virus. Influenza A is the most common, followed by influenza B. Both are highly contagious, and their symptoms are similar.

Influenza, also known as the flu, is a viral respiratory illness that is most prevalent during fall and winter months. These viruses can spread when a person with the infection sneezes or coughs and droplets travel to another person’s nose or mouth.

The flu is different from the common cold. It can cause severe illness and worsen some chronic medical conditions, such as asthma, heart disease, and diabetes. In some cases, it can lead to death.

Read this article to learn more about the types of flu and their symptoms and treatments.

Share on Pinterest
by Alfian Widiantono/Getty Images

There are four types of influenza virus.

Influenza A

Influenza A viruses cause seasonal flu epidemics practically every year in the United States. They can infect humans and animals.

Influenza A is the only type that can cause a pandemic, which is a global spread of disease. Bird flu and swine flu pandemics both resulted from influenza A viruses.

An influenza A virus has two surface proteins: hemagglutinin and neuraminidase. These help doctors with classification.

Influenza B

Influenza B viruses can also cause seasonal epidemics that typically only affect humans. There are two lineages of influenza B: Victoria and Yamagata.

Influenza B viruses mutate more slowly than influenza A viruses.

Influenza C

Influenza C viruses cause mild illnesses — they do not appear to cause epidemics.

Influenza D

Influenza D viruses mainly affect cattle and do not seem to infect humans.

Flu resources

For more information and resources to help keep you and your loved ones healthy this flu season, visit our dedicated hub.

Was this helpful?

Flu symptoms can range from mild to severe, and they vary from person to person.

Common symptoms of the flu include:

Some people experience severe symptoms, which can include:

  • chest pain
  • shortness of breath
  • severe pain
  • severe weakness
  • a high fever
  • seizures
  • severe dizziness
  • loss of consciousness

A person who experiences any severe symptom should receive medical attention.

Influenza A and B differ in terms of how common they are.


According to researchers, influenza A viruses are responsible for about 75% of confirmed flu cases, while influenza B viruses are behind approximately 25% of confirmed cases.


Both influenza A and B are highly contagious.

When a person with the flu coughs or sneezes, droplets can enter another person’s nose or mouth, transmitting the illness.

According to the Centers for Disease Control and Prevention (CDC), flu viruses can infect others from up to 6 feet away.

Alternately, a person can catch the flu if they touch a surface contaminated with the flu virus, then touch their own mouth or nose.

The CDC report that people with the flu are the most contagious in the 3–4 days after becoming ill. Symptoms tend to develop 2 days after the illness starts, so a person may pass on the flu before they feel sick.


For a person who is generally healthy, the flu is not typically dangerous. However, it can severely affect certain groups of people, who should seek medical attention as soon as they suspect that they have flu symptoms.

Those most at risk of developing flu complications include:

  • women who are pregnant
  • people with certain chronic medical conditions
  • children younger than 5
  • adults aged 65 or over

Many people believe that influenza A is more severe than influenza B. However, this is not always the case.

A 2014 study concluded that adults hospitalized with influenza A or B tended to have similarly long hospital stays. They also had similar rates of intensive care unit admission and death during hospitalization.

A 2016 study found that the influenza B virus was more likely to cause death in hospitalized children aged 16 or younger.

The researchers also concluded that children aged 10–16 years with this type of virus were more likely to be admitted to intensive care units, compared with those who had influenza A.

Many people find that home remedies can help ease flu symptoms, but prescription antiviral medication may be a good idea for people with a high risk of complications or severe symptoms.

Home remedies

To reduce flu symptoms at home:

  • drink plenty of fluids
  • get plenty of rest
  • take over-the-counter medications such as ibuprofen or acetaminophen to relieve any pain

Antiviral medications

Antiviral medications are available by prescription only. They can shorten the duration of symptoms or prevent complications, such as pneumonia.

Antivirals can especially benefit people with a greater risk of flu complications, including young children, older adults, pregnant women, and people with certain chronic illnesses.

Antiviral medications work best when a person takes them within 1–2 days of symptoms starting.

There are a few different types of antivirals for the flu, including:

  • oseltamivir
  • zanamivir
  • peramivir
  • baloxavir marboxil

These can come in pill, liquid, inhalable powder, or intravenous forms.

The following can help prevent a person from catching or spreading the flu:

  • limiting contact with sick people
  • staying home when ill
  • covering the nose and mouth when sneezing or coughing
  • washing the hands often
  • disinfecting surfaces that may contain flu germs
  • avoid touching the eyes, nose, and mouth
  • wearing a mask when leaving the house

The best method of prevention is to receive a flu vaccination every year. The flu vaccine can come as an injection or a nasal spray.

According to a 2017 study, the vaccine may reduce the risk of in-hospital deaths from the flu, prevent associated intensive care unit hospitalization, and reduce the duration of related hospital stays.

The following are answers to commonly asked questions about influenza.

What is the difference between influenza A and B?

Influenza A is the most common strain and usually causes seasonal flu outbreaks. Influenza B can also cause flu outbreaks, but it mutates more slowly.

How contagious is influenza A?

Influenza A is a highly contagious virus that causes seasonal flu epidemics in the U.S.

Is the seasonal flu influenza A or B?

Typically, seasonal flu outbreaks are influenza A.

What happens if you get influenza A?

A person may experience fatigue, fever, body aches, and cough. While many people recover with home remedies, if their symptoms are not improving, it is a good idea to seek medical attention.

There are four types of influenza virus, and influenza A and B are the most common.

While many people recover from the flu with home remedies, influenza A and B can each cause serious illness and death in people with a high risk of complications.

There is no cure for the flu, but rest and drinking fluids can help ease symptoms. Antiviral medications may also help shorten the duration of the illness.

People who experience severe flu symptoms or any complications should receive medical attention.