Horizontal transmission is the transmission of a bacterial, fungal, or viral infection between members of the same species that are not in a parent-child relationship.

There are two types of horizontal transmission, anterior station and posterior station. In anterior station, transmission occurs via the bite of an infected organism, 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.

Horizontal transmission tends to evolve virulence. It is therefore a critical concept for evolutionary medicine.