Understanding the Different Types of Flu

Influenza is a respiratory infection caused by one of four different virus types: A, B, C, and D. Influenza is common worldwide, but only influenza types A and B cause seasonal epidemics.

Influenza epidemics sometimes become pandemics, causing mass outbreaks of illness in humans around the world in a relatively short amount of time. In the past, some flu pandemics have caused very severe illness and killed millions of people. Others have been less serious.

This article discusses the four different types of influenza virus and some of the most serious strains.

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Types of Influenza

Influenza is caused by one of three different viruses, named A, B, C, and D. The most serious human infections are caused by influenza type A. This type is more likely to cause an epidemic and is the only one that is known to cause pandemics.

In the United States, influenza caused between 9 million and 41 million illnesses and between 12,000 and 52,000 deaths between 2010 and 2020.

When new flu strains make headlines, they're often referred to by more complicated alphanumeric names like H1N1. Those names are classifications based on subtypes, genetic lineages, and strains.

different types of flu
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What Type Causes the Seasonal Flu?

Three types of flu viruses—A, B, and C—cause seasonal influenza in humans. Influenza D may also have the potential to do so if the potential for future transmission from animals to humans comes to fruition.

Seasonal flu is the type of influenza that typically causes illness for just a few months out of the year. Flu season is different depending on where you are in the world. In the United States, it usually falls between October and April.

Influenza A

Type A influenza is usually responsible for the majority of seasonal flu cases and typically causes the most severe cases. It is found in humans and in animals. An influenza A infection may last for about one to two weeks.

Most of the flu names you're likely familiar with are types of influenza A. Influenza A comes in two subtypes and many strains. The subtypes are classified by the specific combination of two proteins that are attached to them:

  • H, for hemagglutinin protein
  • N, for neuraminidase protein

Only about 66% of the possible combinations of H and N have been found in nature.

Influenza A viruses tend to mutate rapidly. Scientists have to predict their course so they can create the right vaccine to protect people from the next mutated form.

Influenza B

Type B flu is found primarily in humans. It has the potential to be very dangerous, but cases are typically less severe than with influenza A. Like the A virus, illness caused by influenza B may persist for a week or two.

Influenza B viruses can cause epidemics but not pandemics (spread of infection across large parts of the planet).

Influenza B is classified by genetic lineage as opposed to subtype:

  • B/Yamagata
  • B/Victoria

Influenza C

Type C flu, which affects only humans, is much milder than types A and B. It typically causes mild respiratory illnesses and it is not known to have caused any seasonal flu epidemics.

Most people who contract influenza C will experience symptoms similar to those of a cold. Even so, influenza C can become serious in:

  • Infants
  • Elderly people
  • People with severely compromised immune symptoms

In healthy people, influenza C usually goes away on its own in three to seven days. Influenza C outbreaks can sometimes co-exist with influenza A pandemics.

Influenza D

In 2011, influenza D virus was isolated from swine and cattle. It's been reported in multiple countries, suggesting worldwide distribution.

To date, the influenza D virus has not demonstrated the ability to be passed from animals to humans, although scientists suggest that such a jump may be possible.

Type Who Gets It Infectiousness Case Severity Distribution
A Humans and animals High Potentially high Can cause pandemics
B Humans High Somewhat less severe than A Can cause epidemics
C Humans Less than A & B Typically mild Doesn't cause epidemics
D Cattle, swine Low Typically mild Believed to be worldwide

H1N1 Swine Flu

In the spring of 2009, scientists discovered a new influenza A virus in Mexico and named H1N1. It is also known as swine flu.

H1N1 influenza is a combination of human, swine, and bird flu. It became the first flu pandemic the world had seen in more than 40 years. It's technically an influenza A virus but is so mutated that it's not the same as influenza A which causes seasonal flu.

Research suggests that the influenza H1N1 may not be as new as some have suggested. Genetic analyses have linked it to the 1918 flu pandemic which killed over 50 million people, including 675,000 in the United States.

H5N1 Bird Flu

H5N1 is the strain of influenza A known as the bird or avian flu. Typically, it's transmitted between birds, but it can be passed from bird to human. It does not appear to spread from person to person.

When it does infect humans, bird flu can cause very serious illness, though mild and asymptomatic cases have also been reported. Bird flu has killed more than half of the people who have been infected with it.

There is currently no available vaccine for bird flu and the annual flu shot does not protect against it. However, most people are at low risk for bird flu since it tends to only infect people who have close contact with poultry.

Although the risk of contracting bird flu is low, healthcare providers have grave concerns about the potential of H5N1 to mutate and cause a worldwide pandemic.

What About the Stomach Flu?

What many call the stomach flu is actually not flu at all. Rather, it is gastroenteritis, which is in no way related to the influenza virus. Influenza is a respiratory illness. While it can cause vomiting and diarrhea, especially in children, it is always accompanied by respiratory symptoms.

How Influenza Spreads

Influenza is spread from person to person by people who are already infected.

Touching objects the infected person has touched (doorknobs, faucets, phones) or even being in the same room as the person, especially if they are coughing or sneezing, is enough to become infected yourself.

What Types of Flu Do Vaccines Prevent?

Flu vaccines are customized each year to protect against the strains researchers believe are most likely to circulate among humans that season. Every year, the vaccine contains:

  • One influenza A virus (H1N1)
  • One influenza A virus (H3N2)
  • One or two influenza B viruses

The vaccine only protects you from the specific strains it contains. It doesn't contain C or D viruses, and it doesn't protect against other viral illnesses with similar symptoms, which often spread during flu season.

The Centers for Disease Control and Prevention (CDC) recommends that everyone over six months old be vaccinated against the flu unless certain health problems prevent it.

Summary

Influenza is a respiratory infection caused by one of four different viruses: A, B, C, and D. Only A, B, and C are known to cause illness in humans. Influenza A is the only one that has caused global pandemics. Swine flu and bird flu are both types of influenza A.

The best way to protect yourself from influenza is to receive an annual flu shot.

11 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Ghebrehewet S, MacPherson P, Ho A. Influenza. BMJ. 2016;355:i6258. doi:10.1136/bmj.i6258

  2. Centers for Disease Control and Prevention. Disease burden of flu.

  3. Centers for Disease Control and Prevention. Types of influenza viruses.

  4. Nakatsu S, Murakami S, Shindo K, et al. Influenza C and D viruses package eight organized ribonucleoprotein complexes. J Virol. 2018;92(6):e02084-17. doi:10.1128/JVI.02084-17

  5. Bourret V, Lyall J, Frost SDW, et al. Adaptation of avian influenza virus to a swine host. Virus Evol. 2017;3(1):vex007. doi:10.1093/ve/vex007

  6. Jester B, Uyeki TM, Jernigan DB, Tumpey TM. Historical and clinical aspects of the 1918 H1N1 pandemic in the United States. Virology. 2019;527:32-37. doi:10.1016/j.virol.2018.10.019

  7. Mahase E. H5N1: Has there been human-to-human transmission, and do we have a vaccine? BMJ. 2023;380:510. doi:10.1136/bmj.p510

  8. Centers for Disease Control and Prevention. Technical report: Highly pathogenic avian influenza A(H5N1) viruses.

  9. World Health Organization. Human infection with avian influenza A(H5) viruses.

  10. Centers for Disease Control and Preventions. Key facts about flu vaccines.

  11. Centers for Disease Control and Prevention. Who should & who should NOT get a flu vaccine.

Additional Reading

By Kristina Herndon, RN
Kristina Herndon, BSN, RN, CPN, has been working in healthcare since 2002. She specializes in pediatrics and disease and infection prevention.