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Each year, millions of people worldwide get sick with seasonal influenza, or the seasonal flu. What you may not realize is that two main flu virus types make people sick — influenza A and influenza B.

The differences in influenza A and B can matter for what to expect if you get sick.

What Are the Different Types of Flu Viruses?

There are four main types of influenza viruses: A, B, C, and D. Each has different subtypes or lineages.

The biggest public health concerns come from flu A and B. Only these types cause seasonal epidemics — an increase in flu cases above normal levels at certain times of the year.

Flu C causes mild illness in people. Flu D viruses mainly affect cows but don’t make people sick.

Subtypes of flu A

Infectious disease experts divide flu A into subtypes based on two surface proteins:

• Hemagglutinin — 18 subtypes (H1 through H18).
• Neuraminidase — 11 subtypes (N1 through N11).

Researchers have identified more than 130 influenza A subtype combinations. The most common influenza A virus subtypes now spreading in people are A(H1N1) and A(H3N2). The current A(H1N1) strain — the swine flu — is the same type that emerged in the spring of 2009 and caused a flu pandemic (global epidemic of flu disease).

Subtypes of flu B

Experts divide flu B into two main lineages:

• B/Victoria
• B/Yamagata

These strains are less likely to mutate or change as much from year to year as flu A strains.

How Do Influenza A and B Affect Humans?

The flu is a highly contagious respiratory illness that occurs when influenza virus droplets get into and infect your nose, throat, or lungs. It spreads when people who are sick with the flu cough or sneeze and other people breathe in the droplets.

How does influenza A affect humans?

Influenza A accounts for some 75% of flu cases worldwide, according to the National Council on Aging. It’s more common in adults than in children and can affect people and animals, such as birds and pigs.

People rarely get sick directly from animals infected with flu A. But some avian and swine flu A variants have caused sporadic human infections and outbreaks, according to the Centers for Disease Control and Prevention (CDC).

How does influenza B affect humans?

Influenza B accounts for some 25% of all flu cases worldwide. It only affects people and is more common in children than adults.

Flu B spreads later in the flu season, peaking in the spring.

Is Influenza A More Contagious than Influenza B?

Influenza A viruses are the most contagious and can cause flu pandemics. Although flu B can cause outbreaks, it’s less likely to cause pandemics.

Why is flu A worse than flu B?

All flu viruses can mutate or change their genetic makeup, which impacts how quickly they spread and how your body responds.

But flu A is worse than flu B because:

• Influenza A viruses tend to swap their genetic information more often and quickly. People may have little to no immunity against a new strain, which can make fighting off the viral infection harder.
• Influenza B viruses tend to change their genetic makeup more slowly, so your immune system has more time to adapt and respond.

Getting flu A and B at the same time is possible. Sometimes, you can recover from flu A and get flu B right away, or vice versa. Antibodies from one type of flu — proteins your immune system makes to recognize and fight viruses and other infections — don’t protect you from another type.

You can also have the flu with other respiratory illnesses, such as COVID-19 or strep throat.

What Are the Symptoms of Influenza A and B?

Flu symptoms can take a toll on your body. Both flu viruses cause similar symptoms that often come on suddenly. You may feel fine one day and then really sick the next day.

Common early signs of flu include:

• Fever (100.4 F or higher) and/or chills. (Not everyone with flu will develop a fever.)
• Headaches.
• Muscle and body aches.
• Runny or stuffy nose.
• Sore throat and/or coughing.
• Sneezing.
• Weakness, fatigue, or extreme tiredness.

In children, diarrhea and vomiting are also common flu symptoms.

For most people, flu symptoms last anywhere from a few days to two weeks. In some people, the flu can cause serious complications, such as pneumonia and even death.

Is Influenza A or B More Severe?

You may wonder if there’s a difference in how severe influenza A is versus influenza B. Some of it depends on your age and overall health:

• In adults, influenza A can cause more severe illness.
• In children under age 5, influenza B can cause more severe illness.

In general, both types of flu viruses hit older adults and those with chronic health conditions, such as heart disease, the hardest.

What Are the Treatment Options for Influenza A and B?

No matter the type of flu you have, doctors use antiviral medicines to treat flu. These drugs are most effective when taken within 48 hours of flu symptoms starting.

Antibiotics, which only work on bacterial infections, don’t work on the flu.

Antiviral drugs help:

• Prevent serious complications in those at risk for serious illness, including older adults and those with underlying health conditions.
• Reduce symptom severity.
• Shorten how long symptoms last.

The U.S. Food and Drug Administration has approved four antiviral drugs to treat the flu. At-home flu remedies include:

• Drinking plenty of fluids.
• Getting plenty of rest.
• Taking over-the-counter (OTC) medication to reduce fever and relieve pain.

How Can I Prevent Influenza A and B?

Even though flu viruses can spread throughout the year, doctors often warn about seasonal flu. That’s when influenza A and B viruses spread the most and get the most people sick.

In the United States, the fall and winter months are flu season. Peak flu activity happens between December and March, according to the CDC.

The seasonal flu vaccine is the most effective flu prevention for both influenza A and B. Each year, researchers develop a vaccine to target the strains of flu A and B they expect will spread during the upcoming flu season. Because these strains may change, you need to get a flu shot every year, before the next flu season starts.

The CDC recommends that everyone ages 6 months and older get an annual flu vaccine.

When should you get the flu vaccine?

Timing matters when it comes to vaccine protection against the flu. It takes about two weeks for the vaccine to develop antibodies that protect against the flu. For maximum efficacy, health experts recommend getting your flu vaccine no later than October.