Types of Flu Viruses: A Comprehensive Guide to Influenza A, B, and More

Influenza, commonly referred to as the “flu”, is a family of viruses that includes several types. Not all flu viruses are the same. In fact, there are four main types: A, B, C, and D. This article will outline each type - what it is, who it affects, and why it matters.

Influenza A viruses are responsible for most seasonal outbreaks and can affect people of all ages. Influenza B viruses tend to cause milder symptoms and primarily affect children and young adults. Influenza C viruses are less common and typically cause mild illness in older children and adults. Influenza D viruses have not been detected in humans.

For more information on influenza, visit our blog:  Everything You Need to Know About the Flu.

What Are the Types of Flu Viruses?

4 main types of flu viruses: Influenza A, B, C, and D.

• Influenza A and B are the most common and are responsible for seasonal flu outbreaks that affect humans.

• Influenza C generally causes mild illness and usually affects older children and adults.

• Influenza D is found mainly in animals and has not been known to affect humans.

Understanding these flu types is important because it helps explain the differences in flu symptoms, the severity of outbreaks, and how vaccines are developed. Knowing which virus is circulating can also guide prevention and treatment strategies.

Influenza A

Influenza A is the only flu type known to cause pandemics, which highlights its potential for widespread transmission. Symptoms of influenza A are usually the most severe, and include fever, cough, sore throat, runny or stuffy nose, body aches, headache, and fatigue.

Subtypes of Influenza A are classified based on two surface proteins: haemagglutinin (H) and neuraminidase (N). In Australia, the most common subtypes of Influenza A are H1N1 and H3N2. Influenza A virus frequently mutates, making it the most closely monitored flu type, with scientists working tirelessly to stay ahead of its evolution. Due to virus mutation, Influenza A subtypes vary from year to year, influencing seasonal vaccine formulation.

Prevention is key in avoiding the flu. Practice good hygiene by washing your hands frequently, covering your mouth when coughing, and avoiding close contact with others who are sick. The Australian Government's Department of Health recommends that individuals receive an annual flu vaccination to protect against the most prevalent Influenza strains.

If you're concerned about your risk of contracting Influenza A, are showing symptoms, or have questions about vaccination, consult a doctor at Myhealth for further guidance. You can book either a face-to-face consultation or a telehealth appointment to discuss your options.

Influenza B

Influenza B is one of the main types of flu viruses and only infects humans. Although it generally causes milder illness than some Influenza A strains, it’s still a significant contributor to seasonal flu outbreaks and is included in the annual flu vaccine to help protect the community.

There are two lineages of Influenza B: B/Victoria and B/Yamagata. These are not referred to as "subtypes" like in Influenza A, because they are distinct genetic groups. Influenza B virus mutates more slowly than Influenza A, however the virus still changes over time. These changes mean that new strains can appear. Health authorities monitor disease activity closely and decide which two Influenza B strains to include in the annual flu vaccination.

The symptoms of Influenza B are similar to Influenza A, but often less severe, and can include fever, cough, sore throat, body aches, and fatigue, sometimes leading to complications like ear infections or pneumonia.

Annual vaccination remains the best way to protect yourself and those around you—even for healthy individuals. This is especially important in children and adolescents, who are more likely to become unwell and spread the virus to others.

Influenza C

Influenza C is a type of flu virus that causes mild respiratory illness, usually resembling a common cold. Symptoms may include a runny nose, sore throat, or low-grade fever, and most people recover quickly without complications.

Unlike Influenza A and B, Influenza C affects humans, pigs, dogs, and cattle, but it does not spread widely or cause seasonal epidemics. Because of its mild nature and limited impact, Influenza C is not tracked through routine flu surveillance in Australia or globally.

It also does not form part of the annual flu vaccine, which focuses on the more severe and widespread Influenza A and B viruses.

While infections can occur at any age, they are most common in children. No specific treatment or vaccine is needed for Influenza C.

Influenza D

Influenza D is a type of flu virus that primarily infects cattle and pigs. It was first identified in 2011 and is still being studied. At present, there is no evidence that Influenza D infects humans or causes illness in people.

However, because it is a flu virus found in animals, scientists monitor it closely for any signs of zoonotic transmission—which means the potential for the virus to jump from animals to humans. Monitoring helps researchers stay ahead of any possible future risks.

Influenza D is not involved in seasonal flu outbreaks in humans and is not included in the annual flu vaccine. The seasonal flu vaccine only protects against Influenza A and B, which are the main causes of flu in people.

For now, Influenza D remains an animal health concern, not a public health issue for humans.

Influenza A vs B vs C vs D: What’s the Difference?

This section provides a simple comparison of the four known types of influenza viruses: A, B, C, and D. While all these viruses belong to the same influenza family, they differ in several key ways, including how they spread, who they infect, and their impact on public health.

The table below highlights the key differences between these virus types, focusing on which ones are relevant to seasonal flu, vaccine development, and day-to-day risk for people. Influenza A and B are the two main types responsible for yearly flu outbreaks in humans. These are the strains included in the annual flu vaccine to protect the public.

In contrast, Influenza C and D pose less concern for the general population, as they don’t cause significant seasonal flu outbreaks. Most people won’t encounter these types in any meaningful way. Understanding these differences can help you better understand the flu and how to protect yourself.

How Are Flu Viruses Named?

Flu viruses are named using a standard system established by global health authorities like the WHO and CDC. The naming format includes the virus type (e.g., A or B), followed by the virus's location of discovery, the strain number, followed by the year it was identified. The naming system helps track flu strains across different regions and seasons, making it easier for health experts to monitor and respond to outbreaks.

Naming Format

Flu viruses follow a specific naming system that helps health experts track and identify different strains across seasons and regions. Here’s how it works:

1. Type

The Influenza virus antigenic type.

• Example: A, B, C, D

2. Geographic Origin

The location where the virus was first discovered.

• Example: Hong Kong, California.

3. Strain Number

A unique identifier for the strain

• Example: 7, 15.

4. Year of Identification

The year the virus was first identified.

• Example: 2009, 1968.

5. Subtype (for Influenza A only)

Details that further describe specific strains of Influenza A, such as H1N1 or H3N2. These are provided in parentheses.

For example: A/Hong Kong/1/1968(H3N2) refers to a strain of Influenza A first identified in Hong Kong in 1968.

This system helps health professionals track flu strains and determine which ones should be included in the annual flu vaccine.

Why Knowing the Difference Matters

Understanding the different types of flu viruses is essential for several reasons. Flu types influence how outbreaks are tracked, how vaccines are developed, and how public health guidelines are set. The most common seasonal flu cases are caused by Influenza A and B, both of which are included in the annual flu vaccine.

Knowing the difference helps you make sense of flu-related news and vaccine updates, ensuring you're informed about which strains are circulating. It also helps you assess your personal risk, especially if you're in a high-risk group. By staying informed, you can take the right precautions to protect your health.

What Else Should You Know?

Common Symptoms

Flu symptoms can range from mild to severe and typically include:

• Fever or chills

• Cough

• Sore throat

• Runny or stuffy nose

• Muscle or body aches

• Fatigue

• Headaches

Symptoms can vary depending on the flu virus type and the individual’s health, with some people experiencing more severe symptoms than others. It’s important to recognize flu symptoms early and seek medical advice if needed.

For more information, check out our Flu Symptoms blog.

High-Risk Groups

Certain groups are more likely to develop serious complications from the flu. These include:

• Elderly individuals (65 years and older)

• Young children, especially those under 5 years

• Pregnant people or those in the postpartum period

• People with underlying health conditions, such as asthma, diabetes, or heart disease

• Immunocompromised individuals, like those undergoing cancer treatment or with weakened immune systems

If you or a loved one is in one of these high-risk groups, it's important to take extra precautions, such as getting vaccinated and seeking prompt medical advice if flu symptoms develop.

Vaccination for the high-risk groups listed above is funded by the Australian Government for patients who hold a valid medicare card.

For more details, visit our Flu in the Elderly page.

How to Prevent the Flu

The most effective way to prevent the flu is by getting the annual flu vaccine. It helps protect you from the most common strains circulating each season.

In addition to vaccination, these simple practices can help reduce the spread of the flu:

• Wash your hands regularly with soap and water.

• Cover your mouth and nose with a tissue or elbow when coughing or sneezing.

• Avoid close contact with others when you're feeling unwell.

• Stay home if you’re sick to prevent spreading the flu.

These steps, along with vaccination, can significantly lower your risk of getting and spreading the flu.

Vaccination is funded by the Australian Government for medicare-eligible high risk individuals. High risk individuals include people between 6 months and less than 5 years old, ages 65 or older, Aboriginal and/or Torres Strait Islander people over 6 months of age, pregnant people, and individuals with at-risk health conditions (e.g., asthma, diabetes, heart disease).

For further information about eligibility for government funded flu vaccination, contact your local Myhealth clinic. For more information on where to get vaccinated, visit Flu Vaccination Services.

Treatment Options

In most cases, the flu can be managed at home with plenty of rest, fluids, and over-the-counter medications to relieve symptoms. However, if you're at high risk or your symptoms are severe, your doctor may prescribe antiviral medications like oseltamivir (Tamiflu). These can help reduce the severity and duration of the flu if taken early.

It’s important to seek urgent medical advice if you experience difficulty breathing, chest pain, or if symptoms worsen. Most healthy individuals recover from the flu without needing prescription medication, but prompt care is essential for high-risk groups.

When to Speak to a Doctor

If you're unsure which flu strain you may have or whether you need testing or treatment, it's important to speak with your GP. Symptoms can vary in severity depending on the flu type, and high-risk groups (such as the elderly, young children, and pregnant individuals) are more vulnerable to complications.

Your GP can provide personalised advice on whether you need treatment, how to manage your symptoms, and guide you on prevention and vaccination. If you're unsure whether to get vaccinated, your GP can help clarify this based on your individual needs.

If you're looking for a flu consultation or flu vaccination, you can also book an appointment at your nearest Myhealth clinic, where flu vaccinations are available.

FAQs About Flu Types

How many types of flu viruses are there?

There are four main types of flu viruses: Influenza A, Influenza B, Influenza C, and Influenza D. Influenza A and B are responsible for seasonal flu outbreaks in humans and are included in the flu vaccine. Influenza C causes mild illness and is less common, while Influenza D affects cattle and pigs, with no evidence it infects humans.

Which type of flu is the most serious?

Influenza A is generally considered the most serious flu type, as it is responsible for most seasonal flu outbreaks and can lead to pandemics. It is highly contagious and can cause severe illness, especially in vulnerable groups like the elderly, young children, and pregnant people. Influenza B also contributes to seasonal outbreaks but tends to cause less severe illness.

What’s the difference between H1N1 and Influenza A?

H1N1 is a subtype of Influenza A, which caused the 2009 "swine flu" pandemic.

Influenza A viruses are named and further classified by their surface proteins (hemagglutinin and neuraminidase), e.g., H1N1 or H3N2.

Why do flu vaccines change every year?

Flu viruses mutate rapidly, so new strains are frequently emerging. To provide the best protection, flu vaccines are updated annually to include the most prevalent strains expected to circulate and cause infection. This helps ensure the vaccine is effective against the current flu season’s dominant strains of Influenza A and Influenza B.

Are all flu strains tracked by health authorities?

Health authorities, like the WHO, primarily track Influenza A and B strains, as they are responsible for most seasonal flu outbreaks and pandemics. Other strains, like Influenza C and D, are monitored less closely, as they don’t significantly impact public health.

Can animals give humans the flu (zoonotic flu)?

Yes, animals can sometimes transmit flu viruses to humans. This is known as zoonotic flu. For example, Influenza A(H1N1) was originally transmitted from pigs to humans.

For more details, visit WHO - Zoonotic influenza page.