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Types of Influenza Viruses

This is a picture of an influenza virus. Influenza A viruses are classified by sub types based on the properties of their agglutinate (H) and neurasthenia (N) surface proteins. There are 18 different HA sub types and 11 different NA sub types. Sub types are named by combining the H and N numbers – e.g., A(H1N1), A(H3N2). Click on the image to enlarge the picture. resize icon View Larger

There are four types of influenza viruses: A, B, C and D. Human influenza A and B viruses cause seasonal epidemics of disease (known as the flu season) almost every winter in the United States. Influenza A viruses are the only influenza viruses known to cause flu pandemics, i.e., global epidemics of flu disease. A pandemic can occur when a new and very different influenza A virus emerges that both infects people and has the ability to spread efficiently between people. Influenza type C infections generally cause mild illness and are not thought to cause human flu epidemics. Influenza D viruses primarily affect cattle and are not known to infect or cause illness in people.

Influenza A viruses are divided into sub types based on two proteins on the surface of the virus: agglutinate (H) and neurasthenia (N). There are 18 different agglutinate sub types and 11 different neurasthenia sub types (H1 through H18 and N1 through N11, respectively). While there are potentially 198 different influenza A sub type combinations, only 131 sub types have been detected in nature. Current sub types of influenza A viruses that routinely circulate in people include: A(H1N1) and A(H3N2). Influenza A sub types can be further broken down into different genetic “clade” and “sub-clade.” See the “Influenza Viruses” graphic below for a visual depiction of these classifications.

Subtypes of Influenza A Viruses

Influenza A viruses are divided into sub types on the basis of two proteins on the surface of the virus: agglutinate (HA) and neurasthenia (NA)image icon. There are 18 known HA sub types and 11 known NA sub types. Many different combinations of HA and NA proteins are possible. For example, an “H7N2 virus” designates an influenza A virus sub type that has an HA 7 protein and an NA 2 protein. Similarly, an “H5N1” virus has an HA 5 protein and an NA 1 protein.

All known sub types of influenza A viruses can infect birds, except sub types H17N10 and H18N11, which have only been found in bats. Only two influenza A virus sub types (i.e., H1N1, and H3N2) are currently in general circulation among people. Some sub types are found in other infected animal species. For example, H7N7 and H3N8 virus infections can cause illness in horses, and H3N8 virus infection cause illness in horses and dogs.

Lineages of Influenza A Viruses

Avian influenza (AI) viruses – influenza viruses which infect birds –have evolved into distinct genetic lineages in different geographic locations. These different lineages can be distinguished by studying the genetic make-up of these viruses. For example, AI viruses circulating in birds in Asia, called Asian lineage AI viruses, can be recognized as genetically different from AI viruses that circulate among birds in North America (called North American lineage AI viruses). These broad lineage classifications can be further narrowed by genetic comparisons that allow researchers to group the most closely related viruses together. Thus, the North American lineage of H7N9 viruses could be further broken down into the North American ‘wild bird’ lineage versus the North American ‘poultry’ lineage. The host, time period and geographical location are often used in the lineage name to help further delineate one lineage from another.

What Is the Flu?

Flu, or influenza,is a contagious respiratory infection caused by a variety of flu viruses. Symptoms of flu involve muscle aches and soreness, headache, and fever.

How Does a Flu Virus Make Me Sick?

Flu viruses enter the body through the mucus membranes of your nose, eyes, or mouth. Every time you touch your hand to one of these areas, you are possibly infecting yourself with a virus.

This makes it very important to keep your hands germ-free with frequent and thorough hand washing. Encourage family members to do the same to stay well and prevent flu.Continue Reading Below

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What Are the Different Types of Flu?

There are three types of flu viruses: A, B, and C. Type A and B cause the annual influenza epidemics that have up to 20% of the population sniffling, aching, coughing, and running high fevers. Type C also causes flu; however, type C flu symptoms are much less severe.

The flu is linked to between 3,000 and 49,000 deaths and 200,000 hospitalizations each year in the United States. The seasonal flu vaccine was created to try to avert these epidemics.

What Is Type A Flu Virus?

Type A flu or influenza A viruses are capable of infecting animals, although it is more common for people to suffer the ailments associated with this type of flu. Wild birds commonly act as the hosts for this flu virus.

Type A flu virus is constantly changing and is generally responsible for the large flu epidemics. The influenza A2 virus (and other variants of influenza) is spread by people who are already infected. The most common flu hot spots are those surfaces that an infected person has touched and rooms where he has been recently, especially areas where he has been sneezing.

What Is Type B Flu Virus?

Unlike type A flu viruses, type B flu is found only in humans. Type B flu may cause a less severe reaction than type A flu virus, but occasionally, type B flu can still be extremely harmful. Influenza type B viruses are not classified by sub type and do not cause pandemics.

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