Even the tiniest living things may be parasites. The micro-organisms that cause malaria and sleeping sickness, for example, are parasites that need more than one host to complete their life cycles. The diseases are spread by infected insects, which bite human beings to feed on their blood and in so doing pass on the infection. The organisms multiply in the person’s body, causing illness. The cycle is completed when an infection-free insect bites the person and in its turn becomes a carrier of the disease.
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 covid-19, HIV-IDS, SARS, Ebola, swine, 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 are then absorbed by human mucous membranes in the nose, mouth and eyes. Note: the document used as a reference does not use the words urine or saliva so this citation is questionable. A host shifting event is defined as a strain that was previously zoonotic and now circulates exclusively among humans.
Similarity between species, for example, transfer between mammals, is believed to be facilitated by similar immunological defenses. Other factors include geographic area, interspecies behaviours, and phylogenetic relatedness. Virus emergence relies on two factors: initial infection and sustained transmission.
A parasite is a living thing that benefits from a relationship with another species but actually causes harm to that species. Some fungi are found on dying birch trees and can also live for a while on the wood after the tree has died.
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