Horizontal transmission is the transmission of a bacterial, fungal, or viral

Virus

A virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea...

 infection

Infection

An infection is the colonization of a host organism by parasite species. Infecting parasites seek to use the host's resources to reproduce, often resulting in disease...

 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

Malaria

Malaria is a mosquito-borne infectious disease of humans and other animals caused by eukaryotic protists of the genus Plasmodium. The disease results from the multiplication of Plasmodium parasites within red blood cells, causing symptoms that typically include fever and headache, in severe cases...

, dengue fever

Dengue fever

Dengue fever , also known as breakbone fever, is an infectious tropical disease caused by the dengue virus. Symptoms include fever, headache, muscle and joint pains, and a characteristic skin rash that is similar to measles...

, and bubonic plague

Bubonic plague

Plague is a deadly infectious disease that is caused by the enterobacteria Yersinia pestis, named after the French-Swiss bacteriologist Alexandre Yersin. Primarily carried by rodents and spread to humans via fleas, the disease is notorious throughout history, due to the unrivaled scale of death...

. Posterior station is transmission via contact with infected feces. Examples are rickettsiae driven diseases (like typhus

Typhus

Epidemic typhus is a form of typhus so named because the disease often causes epidemics following wars and natural disasters...

), which are contracted by a body louse's fecal material being scratched into the bloodstream.

Horizontal transmission tends to evolve

Evolution

Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...

 virulence

Virulence

Virulence is by MeSH definition the degree of pathogenicity within a group or species of parasites as indicated by case fatality rates and/or the ability of the organism to invade the tissues of the host. The pathogenicity of an organism - its ability to cause disease - is determined by its...

. It is therefore a critical concept for evolutionary medicine

Evolutionary medicine

Evolutionary medicine or Darwinian medicine is the application of modern evolutionary theory to understanding health and disease. It provides a complementary scientific approach to the present mechanistic explanations that dominate medical science, and particularly modern medical education...

.

In Dual Inheritance Theory

Dual inheritance theory

Dual inheritance theory , also known as gene-culture coevolution, was developed in the late 1970s and early 1980s to explain how human behavior is a product of two different and interacting evolutionary processes: genetic evolution and cultural evolution...

, horizontal transmission refers to the spread of cultural traits between members of the same generation.