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.
Verywell / Gary Ferster
When new flu strains make headlines, they’re often referred to by more complicated alphanumeric names likeH1N1.
Those names are classifications based on subtypes, genetic lineages, and strains.
What pop in Causes the Seasonal Flu?
Three types of flu virusesA, B, and Ccause seasonal influenza in humans.
Flu season is different depending on where you are in the world.
In the United States, it usually falls between October and April.
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.
Influenza A viruses tend to mutate rapidly.
Influenza B
pop in 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 triggered by influenza B may persist for a week or two.
It’s thought that COVID prevention measures may have eliminated it.
Beginning in 2024, it is no longer included in flu vaccines.
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.
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.
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.
Research suggests that the influenza H1N1 may not be as new as some have suggested.
H5N1 Bird Flu
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.
There is currently no available vaccine for bird flu and the annual flu shot does not protect against it.
What About the Stomach Flu?
What many call the stomach flu is actually not flu at all.
Rather, it isgastroenteritis, 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.
What Types of Flu Do Vaccines Prevent?
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.
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.
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