The human immune system uses a number of tactics to fight pathogens. The pathogen ’ s job is to evade the immune system, create more copies of itself, and spread to other hosts. Characteristics that help a virus do its job tend to be kept from one generation to another. Characteristics that make it difficult for the virus to spread to another host tend to be lost .
Take, for model, a virus that has a mutation that makes it peculiarly deadly to its homo host and kills the host within a few hours of infection. The virus needs a raw, healthy server for its descendents to survive. If it kills its host before the host infects others, that mutation will disappear .
One way hosts protect themselves from a virus is to develop antibodies to it. Antibodies lock onto the outer surface proteins of a virus and prevent it from entering master of ceremonies cells. A virus that appears different from other viruses that have infected the host has an advantage : the host has no preexistent exemption, in the form of antibodies, to that virus. many viral adaptations involve changes to the virus ’ mho out airfoil.
Below we look at two extra cases in viral evolution : how evolution occurs in influenza viruses and in the human immunodeficiency virus ( HIV, the virus that causes AIDS ). Both of these viruses are RNA viruses, meaning that their familial corporeal is encoded in RNA, not DNA. deoxyribonucleic acid is a more stable molecule than RNA, and DNA viruses have a proofread check as region of their generative process. They manage to use the host cell to verify viral DNA reproduction. If the virus makes a error in copying the deoxyribonucleic acid, the host cell can much correct the mistake. deoxyribonucleic acid viruses, consequently, do not change, or mutate, much. RNA, however, is an fluid atom, and RNA viruses don ’ t have a built-in proofread step in their replication. Mistakes in copying RNA happen frequently, and the host cell does not correct these mistakes. RNA virus mutations are frequent and can have crucial consequences for their hosts .
Influenza viruses are simple entities belonging to one of three types : A, B, or C. They consist of no more than seven or eight RNA segments enclosed within an envelope of proteins. Mutations in viral RNA and recombinations of RNA from different sources lead to viral development .
Influenza viruses can evolve in a gradual way through mutations in the genes that relate to the viral coat proteins hemagglutinin and neuraminidase ( HA and NA in shorthand ). These mutations may cause the virus ’ s out surface to appear different to a host previously infected with the ancestor breed of the virus. In such a case, antibodies produced by previous infection with the ancestor filter can not efficaciously fight the mutated virus, and disease results. ( Hemagglutinin and neuraminidase lend their beginning initials to flu subtypes. For model, the 2009 influenza pandemic was caused by an influenza A H1N1 virus. ) As mutations accumulate in future generations of the virus, the virus “ drifts ” away from its ancestor striving .
antigenic stray is one argue that new influenza vaccines much need to be created for each influenza season. Scientists try to predict which changes are likely to occur to presently circulating influenza viruses. They create a vaccine designed to fight the bode virus. Sometimes the prediction is accurate, and the influenza vaccine is effective. other times the prediction misses the mark, and the vaccine won ’ t prevent disease .
antigenic shift is a process by which two or more different types of influenza A blend to form a virus radically different from the ancestor strains. The virus that results has a newfangled HA or NA subtype. Antigenic chemise may result in global disease spread, or pandemic, because humans will have few or no antibodies to block infection. however, if the new influenza A subtype does not well pass from person to person, the disease outbreak will be limited .
Antigenic shift occurs in two ways. First, antigenic shift can occur through genetic recombination, or reassortment, when two or more different influenza A viruses infect the lapp host cell and combine their genetic material. Influenza A viruses can infect birds, pigs, and humans, and major antigenic shifts can occur when these virus types combine. For exercise, a pig influenza virus and a human influenza virus could combine in a shuttlecock, resulting in a radically different influenza type. If the virus infects humans and is efficiently transmitted among them, a pandemic may occur .
Second, an influenza A virus can jump from one type of organism, normally a bird, to another type of organism, such as a human, without undergoing major genetic change. If the virus mutates in the human host so that it is easily diffuse among people, a pandemic may result .
In all cases, antigenic lurch produces a virus with a raw HA or NA subtype to which humans have no, or very few, preexisting antibodies. once scientists are able to identify the new subtype, a vaccine can by and large be created that will provide protection from the virus .
Why does antigenic shift occur only with influenza A, and not influenza B and C ? Influenza A is the lone influenza type that can infect a wide kind of animals : humans, waterfowl, early birds, pigs, dogs, and horses. Recombination possibilities, consequently, are very low or nonexistent with influenza B and C.
A pandemic had the potential to occur in the bird influenza outbreaks in 2003 in Asia. An H5N1 influenza A virus spread from infect birds to humans, resulting in unplayful human disease. But the virus has not evolved to be easily spread among humans, and an H5N1 pandemic has not occurred .
The virus that causes Acquired Immune Deficiency Syndrome ( AIDS ) is a highly genetically variable virus, for several reasons. First, it reproduces much more quickly than most early entities. It can produce billions of copies of itself each day. As it makes rapid-fire copies of itself, it normally makes errors, which translate into mutations in its genic code. The more beneficial the mutations are to the virus ’ s survival, the more probably that mutated virus will be to reproduce itself .
Another cause of the variability in HIV results from the virus ’ s ability to recombine and form modern variants within an person. This happens when a host cell is infected with two different variations of HIV. Elements of the two viruses may combine to result in a new virus that is a unique combination of the two parents .
The rapid rate of HIV development has authoritative consequences. HIV can promptly develop resistance to anti-HIV drugs. additionally, targeting a vaccine to a quickly changing virus is challenging. To date, researchers have developed respective campaigner HIV vaccines, but none has performed well enough in clinical trials to warrant licensure. Read the article Development of HIV Vaccines to learn more about the challenges and promise .
Burke, D.S. recombination in HIV : An important viral evolutionary scheme. Emerging infectious Diseases. Sept. 1997 ; 3 ( 3 ) Walter Reed Army Institute of Research. Accessed 01/10/2018 .
CDC. How the influenza virus can change : lurch and stray. Accessed 01/10/2018 .
CDC. Types of influenza viruses. Accessed 01/10/2018 .
University of California Museum of Paleontology. evolution from a virus ’ randomness view. Dec. 2007. Accessed 01/10/2018.
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University of California Museum of Paleontology. human immunodeficiency virus : The ultimate evolver. Accessed 01/10/2018 .
survive update 10 January 2018