Understanding the Different Types of Flu

Even though it's a common illness, confusion abounds about what influenza is and is not. Part of that confusion is due to the many types of flu humans can catch, some of which can cause serious illness and death.

Names of Flu Strains

The four types of flu virus have simple names:

  • Influenza A
  • Influenza B
  • Influenza C
  • Influenza D

When new flu strains make headlines, they're often referred to by more complicated alpha-numeric names like H1N1. The names come from the way different subtypes and strains are identified.

That's because the influenza A virus—the most significant one when it comes to human illness—comes in two subtypes and many strains.

The subtypes are classified by proteins attached to them:

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

Beyond that, the H and N subtypes contain different strains, which are referred to by number.

  • The H subtype has 16 different strains.
  • The N subtype has 9 different strains.

When two strains of the influenza A mix, you end up with names such as "H1N1" or "H3N2."

However, the pandemic H1N1 influenza is different because it was created from a combination of human, swine, and bird flu viruses. Although it is technically an influenza A virus, it is a mutation and therefore not the same as the influenza A that causes seasonal flu.

different types of flu
Verywell / Gary Ferster

Seasonal Flu

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, as does D, which is a newer influenza virus with the potential for future transmission from animals to humans.

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. Influenza A 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.

The viruses classified as influenza A(H3N2) are the ones you hear about that tend to mutate rapidly, which keeps scientists busy trying to predict its course and create the right vaccine to predict people from the next mutated form.

Based on the number of subtypes and strains, influenza A could theoretically have 198 different combinations, but 131 have been identified.

Influenza B

Type B flu is found only in humans. Influenza B has the potential to be very dangerous, but it is typically less severe than influenza A. Influenza B viruses can cause epidemics but not pandemics (spread of infection across large parts of the planet).

Influenza B is typically less severe than influenza A but can still be dangerous. The influenza B viruses currently circulating have been classified into two distinct genetic lineages, known as Yamagata and 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 a 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 Severity Distribution
A Humans and animals High Potentially high Can cause pandemics
B Humans High Somewhat less sever 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

Flu Pandemic

Any flu virus has the potential to become a pandemic, 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.

H1N1 Swine Flu

In the spring of 2009, scientists discovered a new influenza A virus in Mexico and named H1N1 (also known as swine flu). It quickly spread throughout North America and around the world.

H1N1 is a combination of human, swine, and bird flu. It became the first flu pandemic the world had seen in more than 40 years.

Recent 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 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 spread from person to person.

When it does infect humans, bird flu is associated with very serious illness, multi-organ failure, and high death rates. In fact, bird flu has killed more than half of the people who've infected with it.

Although the risk of contracting bird flu is low, doctors have grave concerns about the potential of H5N1 to mutate and cause a worldwide pandemic. Increasing rates of H5N1 infections in Egypt suggest that widespread human-to-human transmission may be possible.

"Stomach Flu"

Influenza is a respiratory illness. Technically, there's no such thing as the stomach flu.

What people are talking about when they refer to stomach flu is actually gastroenteritis, which is in no way related to the influenza virus.

Influenza can cause vomiting and diarrhea, especially in children, but it's accompanied by respiratory symptoms.

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

The vaccine 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.

A Word From Verywell

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.

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Article Sources

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