Horizontal transmission is the spread of infectious diseases from one organism to another through direct or indirect contact. It is more common than vertical transmission, which only occurs from parent to offspring. Pathogens can be transmitted through direct contact, contaminated surfaces, or carriers such as mosquitoes.
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 diseases and basically describes the movement of a pathogen from one organism to another through direct or indirect contact. Horizontal transmission and vertical transmission are the two ways the infection occurs.
Infectious diseases are mainly 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 to multiply.
More common than vertical transmission is horizontal transmission. Vertical transmission covers only those cases where a parent transmits a disease to their offspring through reproduction. The baby can be infected during its time in the womb, during the labor process from the vaginal wall or through the mother’s milk. A father can pass on a genetic disease to his child through vertical transmission, but not an infectious disease.
All other cases of person-to-person transfer of infectious diseases are caused by horizontal transmission. Horizontal transmission from animal to human and vice versa is also possible, and sometimes a human can contract a disease from a host animal that is not adversely affected by the disease it carries. The horizontal transmission mode can be indirect or direct.
Direct transmission occurs when the infection moves from one person to another without using an intermediate step. Examples of direct contact include touching, kissing, and intercourse. Although some pathogens spread through the air from one person to another, technically using the air between them as an indirect route, airborne transmission is also considered direct transmission.
An infection that a person detects by touching contaminated surfaces is transmitted indirectly. Included in the definition of indirect contact is an airborne disease that lands on a surface from which the infection is picked up. The object contaminated by the pathogen is known as a fomite. Food-borne and water-borne illnesses are other examples of indirect disease transmission.
Another way of indirect transmission is through a carrier. A carrier is a living thing as opposed to an inanimate object. The vector picks up a disease from one person and transmits it indirectly to another person. One such vector is the mosquito, which carries malaria by biting an infected person and then another person, thus spreading the malaria parasite.
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