Table of ContentsView AllTable of ContentsTypes of InfluenzaInfluenza AInfluenza BInfluenza CInfluenza DH1N1 Swine FluH5N1 Bird FluHow Influenza SpreadsVaccinesNext in Flu GuideSymptoms of the Flu
Table of ContentsView All
View All
Table of Contents
Types of Influenza
Influenza A
Influenza B
Influenza C
Influenza D
H1N1 Swine Flu
H5N1 Bird Flu
How Influenza Spreads
Vaccines
Next in Flu Guide
Influenza epidemics sometimes becomepandemics, 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.
1:51Click Play to Learn About the Symptoms of the Flu
1:51
Click Play to Learn About the Symptoms of the Flu
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.
Verywell / Gary Ferster
The Flu
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.
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.
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.
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.
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 viruses can cause epidemics but not pandemics (spread of infection across large parts of the planet).
Interestingly, the Yamagata lineage hasn’t been detected since 2020. It’s thought that COVID prevention measures may have eliminated it. Beginning in 2024, it is no longer included in flu vaccines.
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:
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.
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.
In the spring of 2009, scientistsdiscovereda new influenza A virus in Mexico and named H1N1. It is also known asswine flu.
H1N1 influenza is a combination of human, swine, andbird 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.
Does the H1N1 Virus Still Exist?
H5N1 is the strain of influenza A known as thebird 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 itdoesinfect 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?
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:
*Prior to 2024, some vaccines included two 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 fluunless certain health problems prevent it.
Flu Shots: What You Should Know
Summary
The best way to protect yourself from influenza is to receive an annual flu shot.
12 SourcesVerywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.Ghebrehewet S, MacPherson P, Ho A.Influenza.BMJ.2016;355:i6258. doi:10.1136/bmj.i6258Centers for Disease Control and Prevention.Disease burden of flu.Centers for Disease Control and Prevention.Types of influenza viruses.Centers for Disease Control and Prevention.US Will Transition to Trivalent Flu Vaccines for 2024-2025.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.02084-17Bourret 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/vex007Jester 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.2018.10.019Mahase E.H5N1: Has there been human-to-human transmission, and do we have a vaccine?BMJ. 2023;380:510. doi:10.1136/bmj.p510Centers for Disease Control and Prevention.Technical report: Highly pathogenic avian influenza A(H5N1) viruses.World Health Organization.Human infection with avian influenza A(H5) viruses.Centers for Disease Control and Preventions.Key facts about flu vaccines.Centers for Disease Control and Prevention.Who should & who should NOT get a flu vaccine.Additional ReadingAsha K, Kumar B.Emerging influenza D virus threat: What we know so far!J Clin Med. 2019;8(2):192. doi:10.3390/jcm8020192
12 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.Ghebrehewet S, MacPherson P, Ho A.Influenza.BMJ.2016;355:i6258. doi:10.1136/bmj.i6258Centers for Disease Control and Prevention.Disease burden of flu.Centers for Disease Control and Prevention.Types of influenza viruses.Centers for Disease Control and Prevention.US Will Transition to Trivalent Flu Vaccines for 2024-2025.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.02084-17Bourret 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/vex007Jester 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.2018.10.019Mahase E.H5N1: Has there been human-to-human transmission, and do we have a vaccine?BMJ. 2023;380:510. doi:10.1136/bmj.p510Centers for Disease Control and Prevention.Technical report: Highly pathogenic avian influenza A(H5N1) viruses.World Health Organization.Human infection with avian influenza A(H5) viruses.Centers for Disease Control and Preventions.Key facts about flu vaccines.Centers for Disease Control and Prevention.Who should & who should NOT get a flu vaccine.Additional ReadingAsha K, Kumar B.Emerging influenza D virus threat: What we know so far!J Clin Med. 2019;8(2):192. doi:10.3390/jcm8020192
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
Ghebrehewet S, MacPherson P, Ho A.Influenza.BMJ.2016;355:i6258. doi:10.1136/bmj.i6258Centers for Disease Control and Prevention.Disease burden of flu.Centers for Disease Control and Prevention.Types of influenza viruses.Centers for Disease Control and Prevention.US Will Transition to Trivalent Flu Vaccines for 2024-2025.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.02084-17Bourret 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/vex007Jester 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.2018.10.019Mahase E.H5N1: Has there been human-to-human transmission, and do we have a vaccine?BMJ. 2023;380:510. doi:10.1136/bmj.p510Centers for Disease Control and Prevention.Technical report: Highly pathogenic avian influenza A(H5N1) viruses.World Health Organization.Human infection with avian influenza A(H5) viruses.Centers for Disease Control and Preventions.Key facts about flu vaccines.Centers for Disease Control and Prevention.Who should & who should NOT get a flu vaccine.
Ghebrehewet S, MacPherson P, Ho A.Influenza.BMJ.2016;355:i6258. doi:10.1136/bmj.i6258
Centers for Disease Control and Prevention.Disease burden of flu.
Centers for Disease Control and Prevention.Types of influenza viruses.
Centers for Disease Control and Prevention.US Will Transition to Trivalent Flu Vaccines for 2024-2025.
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
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
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
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
Centers for Disease Control and Prevention.Technical report: Highly pathogenic avian influenza A(H5N1) viruses.
World Health Organization.Human infection with avian influenza A(H5) viruses.
Centers for Disease Control and Preventions.Key facts about flu vaccines.
Centers for Disease Control and Prevention.Who should & who should NOT get a flu vaccine.
Asha K, Kumar B.Emerging influenza D virus threat: What we know so far!J Clin Med. 2019;8(2):192. doi:10.3390/jcm8020192
Meet Our Medical Expert Board
Share Feedback
Was this page helpful?Thanks for your feedback!What is your feedback?OtherHelpfulReport an ErrorSubmit
Was this page helpful?
Thanks for your feedback!
What is your feedback?OtherHelpfulReport an ErrorSubmit
What is your feedback?
How to recognize flu symptoms, when to seek treatment, and ways to prevent Illness
How Many Different Types of Flu Are There?
Symptoms of Influenza
How Long Does the Flu Last?
How Long Is the Flu Contagious?
Influenza Treatments: How to Get Better at Home or With Medication
By clicking “Accept All Cookies”, you agree to the storing of cookies on your device to enhance site navigation, analyze site usage, and assist in our marketing efforts.Cookies SettingsAccept All Cookies
By clicking “Accept All Cookies”, you agree to the storing of cookies on your device to enhance site navigation, analyze site usage, and assist in our marketing efforts.
Cookies SettingsAccept All Cookies
