Understanding the Different Types of Flu

Although it is a very common illness, there is a lot of confusion about what influenza is and what it isn't. Did you know that there are several different types of influenza? Even when it comes to seasonal flu, there are three different types, two of which cause serious illness in humans. Learn about all the types of flu, how they are classified and what they mean to you.

Seasonal Flu

Seasonal flu is the type of flu 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. There are three types of flu viruses that cause seasonal influenza—A, B, and C—as well as a newer influenza virus with potential for transmission from animals to humans.

Influenza A

Type A influenza is usually responsible for the majority of seasonal flu 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. There are many different varieties of influenza A that are classified into subtypes H and N and even further into different strains.

H and N subtypes of influenza A are based on the particular proteins that are attached to the virus. There are 16 different types of hemagglutinin (H) proteins and nine different types of neuraminidase (N) proteins. This is how names such as "H1N1" or "H3N2" are acquired.

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.

Influenza B

Type B flu is another type of flu that causes seasonal illness. It 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. With that said, influenza C can become serious in infants, the elderly, and people with severely compromised immune symptoms.

Influenza C Influenza is usually self-limiting in healthy people with recovery occurring in three to seven days. Influenza C outbreaks can sometimes co-exist with influenza A pandemics.

Influenza D

Recently, influenza D virus has been isolated from swine and cattle. It has been reported in multiple countries, suggesting a worldwide distribution. To date, the influenza D virus has not demonstrated zoonotic properties (meaning the ability to be passed from animals to humans), although scientists suggest that such a jump is possible.

Flu Pandemic

Any flu virus has the potential to become a flu pandemic, during which there are 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 are less serious.

H1N1 Swine Flu

In the spring of 2009, a new influenza A virus was discovered in Mexico, called H1N1 (also known as swine flu). It quickly spread throughout North America, the United States 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 is 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 people infected with it.

Although the risk of contracting bird flu is low, there are 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 is possible.

A Word From Verywell

Although many people claim to have the flu when they have symptoms like vomiting and diarrhea, these are most often caused by gastroenteritis. Commonly called the "stomach flu", gastroenteritis is not caused by influenza and is not related to the flu at all. Influenza is a respiratory virus. Even though it can cause vomiting and diarrhea in some people, these symptoms are most common in children.

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