<unk>  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,<unk>  tends to evolve virulence. It is therefore a critical concept for evolutionary medicine.
Horizontal transmission
Vertical transmission
Airborne transmission
Herd immunity
Social contagion
Epizootic
Human-to-human transmission
Adoptive immunity