Horizontal transmission is the transmission of organisms between biotic and/or abiotic members of an ecosystem that are not in a parent-progeny relationship. This concept has been generalized to include transmissions of infectious agents, symbionts, and 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. Biological Pathogen transmission 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. Symbiont transmission Symbiosis describes a relationship in which at least two organisms are in an intimately integrated state, such that one organism acts a host and the other as the symbiont. There are obligate, those that require the host for survival, and facultative symbionts, those that can survive independently of the host. Symbionts can follow vertical, horizontal, or a mixed mode of transmission to their host. Horizontal, or lateral, transmission describes the acquisition of a facultative symbiont from the environment or from a nearby host. The life cycle of the host includes both symbiotic and aposymbiotic phases. The aposymbiotic phase generally begins in the germ cells, and during development, the host organism acquires the symbiont and translocates it to a symbiont-housing organ. The host will release the symbiont before reproduction. Determining the transmission mode of a symbiont requires identifying the key aspects that define transmission. For horizontal transmission one would need to check the host at each life stage for the presence of the symbiont and determine whether the symbiont is released from the host into the environment. In hydrothermal vent tubeworms, release of a symbiont upon host death was demonstrated using a symbiont recruitment plates and fluorescence in situ hybridization (FISH). Each life cycle stage of the tubeworm