Disease transmission

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.

Horizontal and vertical
Disease can be directly transmitted in two ways:

(vectors or fomites that allow the transmission of disease without physical contact).
 * 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
 * Vertical disease transmission – passing a disease causing agent vertically from parent to offspring, such as perinatal transmission.

Transmission, symptoms and survival
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.

Locus
In transmission, a locus is the point on the body where a pathogen enters.
 * In droplet contact and other airborne transmission it is generally the respiratory system
 * In direct physical and indirect contact it is generally through a wound in the skin or through a mucous membrane
 * In fecal-oral transmission, it is through the mouth.
 * In vector borne transmission, it is at the bite or sting of the vector.

Droplet Contact
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. Diseases that are commonly spread by coughing or sneezing include (at least):
 * Bacterial Meningitis
 * Chickenpox
 * Common cold
 * Influenza
 * Mumps
 * Strep throat
 * Tuberculosis
 * Measles
 * Rubella
 * Whooping cough

Viral Droplet Nuclei Transmission
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. Viral diseases that are commonly spread by coughing or sneezing droplet nuclei include (at least):


 * Common cold
 * Influenza A & B
 * Mumps
 * Measles
 * Rubella
 * SARS

Fecal-Oral Transmission
Direct contact is rare in this route, for humans at least. More common are the indirect routes; foodstuffs or water become contaminated (by people not washing their hands before preparing food, or untreated sewage being released into a drinking water supply) and the people who eat and drink them become infected. In developing countries most sewage is discharged into the environment or on cropland as of 2006; even in developed countries there are periodic system failures resulting in a sanitary sewer overflow. This is the typical mode of transmission for the infectious agents of (at least):
 * Cholera
 * Hepatitis A
 * Polio
 * Rotavirus
 * Salmonella

Sexual Transmission
This refers to any disease that can be caught during sexual activity with another person, including vaginal or anal sex or (less commonly) through oral sex (see below). Transmission is either directly between surfaces in contact during intercourse (the usual route for bacterial infections and those infections causing sores) or from secretions (semen or the fluid secreted by the excited female) which carry infectious agents that get into the partner's blood stream through tiny tears in the penis, vagina or rectum (this is a more usual route for viruses). In this second case, anal sex is considerably more hazardous since penis opens more tears in the rectum than the vagina, as the vagina is more elastic and more accommodating.

Some diseases transmissible by the sexual route include (at least):
 * HIV/AIDS
 * Chlamydia
 * Genital warts
 * Gonorrhea
 * Hepatitis B
 * Syphilis
 * Herpes

Oral Sexual Transmission
Sexually Transmitted Diseases such as HIV and Hepatitis B are thought to not normally be transmitted through mouth-to-mouth contact, although it is possible to transmit some STDs between the genitals and the mouth, during oral sex. In the case of HIV this possibility has been established. It is also responsible for the increased incidence of herpes simplex virus 1 (which is usually responsible for oral infections) in genital infections and the increased incidence of the type 2 virus (more common genitally) in oral infections.

Oral Transmission
Diseases that are transmitted primarily by oral means may be caught through direct oral contact such as kissing, or by indirect contact such as by sharing a drinking glass or a cigarette.

Diseases that are known to be transmissible by kissing or by other direct or indirect oral contact include all of the diseases listed above as transmissible by droplet contact and also (at least):


 * Cytomegalovirus infections
 * Herpes simplex virus (especially HSV-1)
 * Infectious mononucleosis

(Notice these are all forms of herpes virus.)

Transmission by direct contact
Diseases that can be transmitted by direct contact are called contagious (contagious is not the same as infectious; although all contagious diseases are infectious, not all infectious diseases are contagious). These diseases can also be transmitted by sharing a towel (where the towel is rubbed vigorously on both bodies) or items of clothing in close contact with the body (socks, for example) if they are not washed thoroughly between uses. For this reason, contagious diseases often break out in schools, where towels are shared and personal items of clothing accidentally swapped in the changing rooms.

Some diseases that are transmissible by direct contact include:
 * Athlete's foot
 * Impetigo
 * Syphilis (on rare occasions, if an uninfected person touches a chancre)
 * Warts

Vertical Transmission
This is from mother to child, often in utero or during childbirth (also referred to as perinatal infection). It occurs more rarely via breast milk.

Infectious diseases that can be transmitted in this way include: HIV, Hepatitis B and Syphilis.

Iatrogenic Transmission
Transmission due to medical procedures, such as injection or transplantation of infected material.

Some diseases that can be transmitted iatrogenically include:
 * Creutzfeldt-Jakob Disease by injection of contaminated human growth hormone.
 * MRSA infection is often acquired as a result of a stay in hospital

Vector borne transmission
A vector is an organism that does not cause disease itself but that transmits infection by conveying pathogens from one host to another.

The route of transmission is important to epidemiologists because patterns of contact vary between different populations and different groups of populations depending on socio-economic, cultural and other features. For example, low personal and food hygiene due to the lack of a clean water supply may result in increased transmission of diseases by the fecal-oral route, such as cholera. Differences in incidence of such diseases between different groups can also throw light on the routes of transmission of the disease. For example, if it is noted that polio is more common in cities in underdeveloped countries, without a clean water supply, than in cities with a good plumbing system, we might advance the theory that polio is spread by the fecal-oral route.