Horizontal transmission is a term used to describe one of the ways a disease passes from one organism to another. It is specific to infectious disease and basically describes the movement of a pathogen from one organism to the next through direct or indirect contact. Horizontal transmission and vertical transmission are the two ways infection occurs.
Infectious diseases are mostly caused by viruses, bacteria, and fungi. Prions, such as in the case of mad cow disease, or Creutzfeld-Jakob Disease (CJD), are another, less common cause. These pathogens all need the ability to jump from one person to another in order to multiply.
More common than vertical transmission is horizontal transmission. Vertical transmission only covers those cases where a parent passes a disease onto her child through reproduction. The child can become infected during his or her time in the womb, during the labor process from the vaginal wall, or through breast milk. A father can pass a genetic disease on to his child through vertical transmission but not an infectious disease.
All other cases of infectious disease transfer from person to person are caused by horizontal transmission. Horizontal transmission from animal to human and vice versa are also possible, and sometimes, a human can contract a disease from an animal host that is not adversely affected by the disease it carries. The horizontal mode of transmission can be indirect or direct.
Direct transmission occurs when the infection moves from one person to another without using an intermediate step. Direct contact examples include touching, kissing, and sexual intercourse. Although some pathogens spread through the air from one person to another, technically using the air between them as an indirect step, airborne transmission is also regarded as direct transmission.
An infection that a person picks up by touching contaminated surfaces is indirectly transmitted. An airborne disease that lands on a surface from whence the infection is picked up is included in the definition of indirect contact. The object the pathogen contaminates is known as a fomite. Food or waterborne diseases are other examples of indirect disease transmission.
Another manner of indirect transmission is through a vector. A vector is a living thing as opposed to an inanimate object. The vector picks up a disease from one person and indirectly gives it to another person. One such vector is the mosquito, which carries malaria by biting an infected person and then biting another person, thereby spreading the malaria parasite.