Horizontal transmission

Horizontal transmission is the transmission of infections between members of the same species that are not in a parent-child relationship. This has been generalized to include transmissions of cultural traits between humans.

Because the evolutionary fate of the agent is not tied to reproductive success of the host, horizontal transmission tends to evolve virulence. It is therefore a critical concept for evolutionary medicine.

In biological, but not cultural, transmissions the carriers (also known as vectors) may include other species. The two main biological modes of transmission are anterior station and posterior station. In anterior station, transmission occurs via the bite of an infected organism (the vector), like in malaria, dengue fever, and bubonic plague. Posterior station is transmission via contact with infected feces. Examples are rickettsiae driven diseases (like typhus), which are contracted by a body louse's fecal material being scratched into the bloodstream. The vector is not necessarily another species, however. For example, a dog infected with Rabies may infect another dog via anterior station transmission. Moreover, there are other modes of biological transmission, such as generalized bleeding in ebola.

Cultural transmission may also be horizontal which is explicitly reified in Dual Inheritance Theory. Horizontal transmission is implicit in the meme theory of cultural evolution, where the "meme" has been characterized by Richard Dawkins as a "Virus of the Mind".

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