Hemagglutinin is a compound that causes red blood cells to coagulate. It is also a molecule found on the surface of the virus that binds to sialic acid groups.
Viral neuraminidase is a substance on the surface of influenza viruses which enables the virus to initiate its releasion from the host cell, and are enzymes required during the influenza virus replication process. During the influenza virus replication, neuraminidase attaches to the interior cell membrane using hemagglutinin. In order to initiate the releasion from the host cell, neuraminidase cleaves the sialic acid groups from host glycoproteins. The cleavage of the sialic groups is an important part of virus replication, applying neuraminidase inhibitors to the virus is an effective solution to treat influenza infection.
There are mainly three types of influenza viruses, A, B, and C. The avian influenza virus falls under type A.
Influenza type A viruses can infect many animals including people, but birds are the primary hosts for influenza viruses. The most contagious influenza virus of humans are Influenza type A viruses, and type A viruses are divided into subtypes based on the two proteins on the surface of the virus, hemagglutinin (HA) and neuraminidase (NA). There are 15 different HA subtypes and 9 different NA subtypes. Among many possible combination of the two proteins, only a small portion is in general circulation among people. Other subtypes are found most commonly in other animal species. For example, H7N7 and H3N8 viruses can cause illness in horses. Influenza A virus are named according to their surface proteins. For example, an "H2N3 virus" is an influenza A subtype that has an HA 2 protein and an NA 3 protein.
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H1N1 is also called swine flu because people who get this disease in the past usually has direct contact with pigs. However, as is become more and more common, people can get this disease easily. In 2009, there was a huge outbreak of H1N1 and many people around the world got sick.
H1N2, commonly referred as bird flu, is a type of high infections avian influenza virus. H1N2 virus is known for its high rate of mutation. This high rate of mutation has made making vaccines particularly for one version of this virus difficult.
H1N2 virus was first identified in China in the 1980s and 1990s. Many isolated cases happened in China during this period, but no further spread was identified. However, in the 2000s, H1N2 strain was detected in many northern hemisphere coutries such as the United States, Canada, and France. The outbreaks were settled, but H1N2 is still among human circulation
Along with H1N1 and H1N2, H3N2 is one of the three influenza virus that circulates among humans. H3N2 virus can be mutated into many different strians, and is a seasonal influenza. It is estimated to be killing more than 35000 people in the United States every year