In medicine and biology, transmission is the passing of a communicable disease from an infected host individual or group to a conspecific individual or group, regardless of whether the other individual was previously infected. Sometimes transmission can specifically mean infection of a previously uninfected host.
The term usually refers to the transmission of microorganisms directly from one person to another by one or more of the following means:
- droplet contact - coughing or sneezing on another person
- direct physical contact - touching an infected person, including sexual contact
- indirect physical contact - usually by touching soil contamination or a contaminated surface
- airborne transmission - if the microorganism can remain in the air for long periods
- fecal-oral transmission - usually from contaminated food or water sources
Transmission can also be indirect, via another organism, either a vector (e.g. a mosquito) or an intermediate host (e.g. tapeworm in pigs can be transmitted to humans who ingest improperly cooked pork). Indirect transmission could involve zoonoses or, more typically, larger pathogens like macroparasites with more complex life cycles.
Disease can be directly transmitted in two ways:
- Horizontal disease transmission – from one individual to another in the same generation (peers in the same age group). Horizontal transmission can occur by either direct contact (licking, touching, biting), or indirect contact air - cough or sneeze (vectors or fomites that allow the transmission of disease without physical contact).
- Vertical disease transmission – passing a disease causing agent vertically from parent to offspring, such as perinatal transmission.
Pathogens must have a way to be transmitted from one host to another to ensure their species' survival. Infectious agents are generally specialized for a particular method of transmission. Taking an example from the respiratory route, from an evolutionary perspective a virus or bacteria that causes its host to develop coughing and sneezing symptoms has a great survival advantage - it is much more likely to be ejected from one host and carried to another. This is also the reason that many microorganisms cause diarrhea.
Also known as the respiratory route, it is a typical mode of transmission among many infectious agents. If an infected person coughs or sneezes on another person the microorganisms, suspended in warm, moist droplets, may enter the body through the nose, mouth or eye surfaces.
Droplet nuclei are an important mode of transmission among many infectious viruses such as Influenza A. When viruses are shed by an infected person through coughing or sneezing into the air, the mucus coating on the virus starts to evaporate. Once this mucus shell evaporates the remaining viron is called a droplet nucleus or quanta. The mucus evaporation rate is determined by the temperature and humidity inside the room. The lower the humidity, the quicker the mucus shell evaporates thus allowing the droplet nuclei to stay airborne and not drop to the ground. The low indoor humidity levels in wintertime buildings ensure that higher levels of droplet nuclei will survive: droplet nuclei are so microscopic that they are able to stay airborne indefinitely on the air currents present within indoor spaces. The Wells-Riley equation predicts the infection rates of persons who shed quanta within a building and is used to calculate indoor infection outbreaks within buildings. When an infected person coughs or sneezes, a percentage of their viruses will become droplet nuclei. If these droplet nuclei gain access to the eyes, nose or mouth of an uninfected person (known as a susceptible) -- either directly, or indirectly after touching a contaminated surface -- then the droplet nuclei may penetrate into the deep recesses of their lungs.
Sneeze Bacteria Transmission
While generally harmless in healthy individuals, sneezes spread disease through the infectious aerosol droplets, commonly ranging from 0.5 to 5 µm. 40,000 droplets can be produced by a sneeze.
Conservative estimates place the speed of release at around 100 mph. However, the data collected from the 147th episode of Mythbusters titled 'Flu Fiction', concluded that the speed is closer to 35-40 mph, and that a sneeze can launch droplets from 15 to 20 feet.
Examples of preventive techniques are: the deep exhalation of the air in the lungs that would otherwise be used in the act of sneezing, holding the breath in while counting to ten, crinkling the nose and keeping the eyes open, or gently pinching the bridge of the nose for several seconds.
Proven methods to reduce sneezing generally advocate reducing interaction with irritants, such as keeping pets out of the house to avoid animal dander; ensuring the timely and continuous removal of dirt and dust particles through proper housekeeping; replacing filters for furnaces and air-handling units; employing air filtration devices and humidifiers; and staying away from industrial and agricultural zones. Some people, however, find sneezes to be pleasurable and would not want to prevent them.
See also: http://en.wikipedia.org/wiki/Transmission_(medicine)
Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License and Creative Commons Attribution-ShareAlike License.)