The seasonal flu viruses that humans face every winter season in the United States are caused by human influenza A. Influenza A viruses are categorized as either the hemagglutinin subtype or the neuraminidase subtype based on the proteins involved, and there are 18 distinct subtypes of hemagglutinin and 11 distinct subtypes of neuraminidase. Influenza A is the primary cause of flu epidemics, and they constantly change and are difficult to predict.
Meanwhile, influenza B viruses also affect humans and can be categorized by different strains and lineages. You may have heard that there are different types of influenza viruses — particularly influenza A and influenza B. Influenza A and B are the two types of influenza that cause epidemic seasonal infections nearly every year.
Influenza A can be found in many species, including humans, birds , and pigs. Due to the breadth of potential hosts and its ability to genetically change over a short amount of time, influenza A viruses are very diverse. Influenza D is found mainly in cattle. Influenza A is further divided into different subtypes. These subtypes are based off of the combination of two proteins on the viral surface: hemagglutinin H and neuraminidase N.
There are 18 different H subtypes and 11 different N subtypes. For example, the most common influenza A subtypes that go around seasonally in humans are H1N1 and H3N2. In , H3N2 spread to dogs in Florida. In , this same strain also infected dogs in an earlier outbreak in Chicago. But it can be broken down further into specific viral lineages and strains. Influenza B infections account for the remaining 25 percent. While most confirmed infections during flu season will be influenza A, the occurrence of influenza B infections can increase late in the flu season.
This happened in the to flu season. Both influenza A and influenza B are extremely contagious. People who get either type can spread the virus to others from up to six feet away when they cough or sneeze. You can also contract the virus by touching a surface that has the virus on it and then touching your nose or mouth.
Treatment is focused around relieving symptoms until your body clears the virus naturally. 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 flu season almost every winter in the United States.
Influenza A viruses are the only influenza viruses known to cause flu pandemics, i. A pandemic can occur when a new and different influenza A virus emerges that both infects people and has the ability to spread efficiently among people. Influenza C virus infections generally cause mild illness and are not thought to cause human epidemics.
Influenza D viruses primarily affect cattle and are not known to infect or cause illness in people. Influenza A viruses are divided into subtypes based on two proteins on the surface of the virus: hemagglutinin H and neuraminidase N. There are 18 different hemagglutinin subtypes and 11 different neuraminidase subtypes H1 through H18 and N1 through N11, respectively. Reassortment can occur when two influenza viruses infect a host at the same time and swap genetic information.
This graphic shows the two types of influenza viruses A and B that cause most human illness and that are responsible for flu seasons each year. Both influenza A and B viruses can be further classified into clades and sub-clades which are sometimes called groups and sub-groups. Note that this graphic is an example, and currently circulating influenza clades and subclades may differ from those presented here. Figure 1 — This is a picture of a phylogenetic tree.
Each sequence from a specific influenza virus has its own branch on the tree. The degree of genetic difference between viruses is represented by the length of the horizontal lines branches in the phylogenetic tree. This causes a higher-than-normal incidence or severity of flu. The H1N1 strain is still active and it makes up part of the flu vaccine developed for this coming year.
The fact that the flu changes from one year to the next is not a cause for concern, especially if you get vaccinated.
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