Microorganisms vary widely in the length of time that they can survive outside the human body, and so vary in how they are transmitted.
In order to survive, microorganisms must have a way to be transmitted from one host to another. 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. Diseases that are commonly spread by coughing or sneezing include (at least):
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):
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 stretchier and more accommodating.
Some diseases transmissible by the sexual route include (at least):
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.
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):
(Notice these are all forms of herpes virus.)
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:
Some diseases that can be transmitted in this way include:
Some diseases that can be transmitted iatrogenically include:
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.