Cross-species transmission (CST), also called interspecies transmission, host jump, or spillover, is the ability for a foreign virus, once introduced into an individual of a new host species, to infect that individual and spread throughout a new host population.  Steps involved in the transfer of viruses to new hosts include contact between the virus and the host,  infection of an initial individual  leading to amplification and an outbreak, and the generation within the original or new host of viral variants that have the ability to spread efficiently between individuals in populations of the new host Often seen in emerging viruses where one species transfers to another , which in turn transfers  to humans. Examples include SARS, Ebola, swine flu, rabies, and avian influenza. Bacterial pathogens can also be associated with CST. The exact mechanism that facilitates transfer is unknown, however, it is believed that viruses with a rapid mutation rate are able to  overcome host-specific immunological defenses . This can occur between species that have high contact rates . It can also occur between species with low contact rates but usually through an intermediary species. Bats, for example, are mammals and can directly transfer rabies to humans through bite and also through aerosolization of bat saliva and urine which can then be absorbed by human mucous membranes in the nose, mouth  and eyes. A host shifting event is defined as a strain that was previously zoonotic and now circulates exclusively among humans. The similarity between species, for example, transfer between mammals, is believed to be facilitated by similar immunological defenses.  Other factors include geographic area, intraspecies behaviors, and phylogenetic relatedness . Virus emergence relies on two factors: initial infection and sustained transmission .
Cross-species transmission (CST), also called interspecies transmission, host jump, or spillover, is the transmission of an infectious pathogen, such as a foreign virus, between hosts belonging to different species. Once introduced into an individual of a new host species, the pathogen may acquire the ability to infect other individuals of the same species, allowing it to spread through the new host population. The concept is most commonly studied in virology, but cross-species transmission may also occur with bacterial pathogens or other types of microorganisms. Steps involved in the transfer of pathogens to new hosts include contact between the pathogen and the host, the successful infection of an initial individual host leading to amplification and an outbreak, and the generation within the original or new host of variants that have the ability to spread efficiently between individuals in populations of the new host . The phenomenon is important for understanding and controlling emerging infectious diseases in humans, especially those caused by viruses, where one reservoir species transfers the virus to a vector species , which in turn transfers the virus to humans. Most viral diseases of humans are zoonotic in origin, having been historically transmitted to human populations from various animal species; examples include SARS, Ebola, swine flu, rabies, and avian influenza. The exact mechanisms that facilitate cross-species transmission vary by pathogen, and even for widespread diseases are often unknown; however, it is believed that viruses with very high mutation rates are able to rapidly adapt to new hosts and thereby overcome host-specific immunological defenses , allowing their continued transmission. This may occur between species which are frequently in contact with each other . It can also occur between species with low contact rates if facilitated by an intermediary species. For example, bats can directly transfer the rabies virus to humans through bites, and also through aerosolization of bat saliva and urine which can then be absorbed by human mucous membranes in the nose, mouth , and eyes. A host shifting event occurs when a strain that was previously zoonotic now circulates exclusively among humans. Similarity between host species influences the likelihood that a pathogen can be transmitted between them; for example, most human zoonotic transmissions come from other species of mammals. This is believed to be facilitated by the similarity of the host species' immunological defenses. Pathogens of more distantly related species, on the other hand, such as plant viruses, are not known to infect humans. Other factors include geographic area, intraspecies behaviors, and phylogenetic relatedness .
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the pathogen may acquire the ability to infect other individuals of the same species, allowing it to spread through the
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The concept is most commonly studied in virology, but cross-species transmission may also occur with bacterial pathogens or other types of microorganisms.
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viruses
pathogens
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Often seen in emerging viruses where one species transfers to another
. The phenomenon is important for understanding and controlling emerging infectious diseases in humans, especially those caused by viruses, where one reservoir species transfers the virus to a vector species
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Examples
Most viral diseases of humans are zoonotic in origin, having been historically transmitted to human populations from various animal species; examples
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Bacterial pathogens can also be associated with CST.
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mechanism that facilitates transfer is unknown,
mechanisms that facilitate cross-species transmission vary by pathogen, and even for widespread diseases are often unknown;
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a rapid mutation rate
very high mutation rates
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rapidly adapt to new hosts and thereby
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, allowing their continued transmission. This may
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which are frequently in contact with each other
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if facilitated by
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Bats, for example, are mammals and
For example, bats
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Similarity between host species influences the likelihood that a pathogen can be transmitted between them;
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most human zoonotic transmissions come from other species of mammals. This
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the similarity of the host species'
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Pathogens of more distantly related species, on the other hand, such as plant viruses, are not known to infect humans.
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. Virus emergence relies on two factors: initial infection and sustained transmission