Horizontal transmission

Because the evolutionary fate of the agent is not tied to reproductive success of the host, horizontal transmission tends to evolve virulence.

In anterior station, transmission occurs via the bite of an infected organism (the vector), like in malaria, dengue fever, and bubonic plague.

Examples are rickettsiae driven diseases (like typhus), which are contracted by a body louse's fecal material being scratched into the bloodstream.

[4] Implications of horizontal transmission include the critical need for specificity in recognition and acquisition methods and the larger genetic diversity maintained by the symbiont species.

[5] An example of a horizontally transmitted symbiont with a high specificity recognition and acquisition method is the Rhizobia-Legume symbiosis.

The establishment of the symbiosis begins with the aposymbiotic plant releasing flavinoids that are detected by a specific Rhizobium species and triggers the induction of nod genes in the bacterium.