If you’ve ever had the flu, you’ll know how terrible it can feel. Influenza A and Influenza B are the two main types that infect humans. While they often cause similar symptoms, there are a few key differences in how they spread, how they evolve, and who they affect most.
Understanding how these viruses work can help you protect yourself and your family, especially during flu season.
In this article, we:
Important Note: This article provides general health information and is not a substitute for professional medical advice. Always consult a qualified healthcare professional for diagnosis and treatment of any health condition.
What we call the flu is a common viral illness that shows up every year, especially during colder months. It usually causes symptoms like fever, chills, body aches, fatigue, and a cough.
For some people, this means a few days of feeling unwell; for others, especially older adults or those with existing health conditions, it can lead to more serious complications.
Even though we usually refer to it as just ‘the flu’, there are actually two main types of flu viruses that infect humans: Influenza A and Influenza B.
They both cause similar symptoms, but they differ in how they spread, how they evolve, and who they affect most. In the next section, we’ll take a closer look at the differences between the two.
Even though Influenza A and B cause similar symptoms, they have important differences in their biology, how common they are, and their potential to cause widespread illness.
One of the biggest differences is who these viruses infect and how quickly they change.
Influenza A viruses are unique because they can infect a wide range of hosts, including humans, birds, pigs, and other animals. This broad range of hosts is a key reason why Flu A can mutate more often and more significantly.
These changes mean our immune system might not fully recognise the virus each year, which is why flu vaccines need to be updated annually. (These changes can also lead to global flu outbreaks, or pandemics.)
Influenza B viruses mostly infect humans. Since influenza B is mainly limited to human hosts, it changes more slowly and is more predictable.
The timing and frequency of outbreaks also vary between the two types.
Generally, Influenza A is more common and makes up a larger percentage of flu cases each year. It usually appears earlier in the flu season, often peaking in the autumn and early winter months in Australia.
Influenza B is less common overall compared to Influenza A. While it can still cause significant outbreaks, it tends to become more widespread later in the flu season, often peaking in late winter or early spring.
Influenza A and B can both make you very unwell, but there are some important differences in how severe they tend to be and how widely they can spread.
Influenza A is linked to more severe illness in adults and is the only type of flu that has caused pandemics. It can infect both humans and animals, which gives it more chances to mutate. These mutations can lead to new strains that our immune systems don’t recognise, making it easier for the virus to spread quickly and on a much larger scale.
Influenza B, on the other hand, primarily spreads among humans, and it changes more slowly over time. That makes it more predictable and far less likely to cause a pandemic.
However, it’s still a serious illness. Flu B can lead to complications, especially in young children, older adults, and anyone with a weaker immune system. While it might not have the same global impact as Flu A, it can still cause a lot of disruption in local communities during flu season.
While both types of flu can affect anyone, there are some patterns in who tends to be more severely impacted.
Influenza A can affect all age groups, but it is often linked to more severe illness and complications in adults.
Influenza B is more commonly associated with severe illness in children, especially children under five years old. This might be because younger children have had less exposure to specific Influenza B lineages and therefore have less natural protection.
Despite their differences, Influenza A and B share many key features.
Influenza A and B cause nearly identical symptoms, so it’s hard to tell them apart without a proper test. Most people will experience:
Stomach symptoms like nausea, vomiting, or diarrhoea can also happen, especially in children, but are less common in adults.
Influenza A and B are both very contagious and spread in the same way, mainly through tiny droplets released when an infected person coughs, sneezes, or talks.
These droplets can then be breathed in by others or land on surfaces, which are then touched and transferred to the mouth, nose, or eyes.
Both Influenza A and B are treated in the same way. If you’re in a high-risk group or feeling really unwell, a doctor might prescribe an antiviral. These can help shorten how long you’re sick and make symptoms a bit milder, but you’d need to start taking them within the first 48 hours of feeling unwell.
For most people, though, it’s about managing symptoms at home. That means:
If your symptoms get worse or you’re not getting better, it’s always best to check in with your doctor.
Influenza A and B are both highly contagious. People can start spreading the virus about a day before they even feel sick, and usually for around five to seven days after symptoms begin.
Kids and people with weaker immune systems can also be contagious for longer than that.
Since the symptoms are so similar, laboratory tests are often needed to confirm which type of influenza you have, and to tell it apart from other illnesses like the common cold or COVID-19.
Getting an accurate diagnosis is important for managing your health and for public health tracking. It helps health authorities understand which flu strains are circulating, which then helps with vaccine development.
There are a few ways doctors can test for the flu and figure out which type you have:
Both types of flu can be managed with the right care and prevented with a few simple steps. Here’s how to treat and prevent it.
As mentioned, antiviral medications are available and can be helpful if taken early in the illness. Beyond antivirals, looking after yourself is key.
This includes:
The most effective way to prevent both Influenza A and B is through vaccination and good hygiene, as recommended by Australian health authorities.
While most people recover from the flu without serious problems, some symptoms or risk factors mean you should see a doctor straight away.
You should get medical help straight away if you or someone you know has:
Some people are more at risk of serious illness from the flu and should see a doctor if they get symptoms, including:
Flu A and B might cause the same symptoms, but they’re not exactly the same. Influenza A spreads more widely, mutates faster, and has the potential to cause pandemics, which is why it’s often more serious in adults. Influenza B tends to move more slowly, mainly affects humans, and it can be harder on kids who haven’t built up immunity yet.
Even though they have their differences, both types are highly contagious and can make you feel incredibly unwell. The best way to protect yourself and those around you is to get your yearly flu shot, practise good hygiene, and stay home if you're sick. Knowing this can help you make better choices and stay prepared each flu season.
Yes, it’s technically possible to catch both Influenza A and B at the same time, but it’s rare. You’d need to be exposed to two different flu viruses at the same time for that to happen.
Yes, the annual flu vaccine is designed to protect against the strains of both Influenza A and B viruses that are expected to circulate during the upcoming flu season.
While Influenza A is often linked to more severe illness and pandemics, Influenza B can hit children harder, since they might not have been exposed to it before. How sick you get depends on the strain, your age, overall health, and how strong your immune system is.
For most healthy individuals, flu symptoms can last about a week. However, some people may experience a lingering cough or fatigue for up to two weeks or more.
© 2025 Updoc. All rights reserved.
