Sources of infection
Infection is caused either by organisms from the host's normal flora (endogenous infection) or by organisms transmitted from another source (exogenous infection).
The normal flora will only invade if circumstances permit, as in some of the following examples.
The most important source of human infection is other humans. Some agents (e.g. measles) are more transmissible than others. An epidemic or outbreak can occur if each infected individual typically transmits the pathogen to more than one additional person.
Animal pathogens may be spread to humans by direct contact or in food. Such infections are called zoonoses and are encouraged by intensive farming methods that permit organisms hazardous to humans to spread within the herd or flock (e.g. bovine tuberculosis or E. coli O157). The risks increase with increasing intensity of farming: battery hens are especially prone to the spread of Salmonella and mechanized meat recovery techniques may increase the likelihood of cross-contamination. Feeding ruminant offal to cattle resulted in an epidemic of bovine spongiform encephalopathy (BSE), which then spread to humans as variant Creutzfeldt- Jakob disease (vCJD). The introduction of good farming and food factory techniques will reduce the risks of zoonoses. Humans can become infected from organisms in the inanimate environment. For example, poorly maintained air-conditioning cooling towers can be a source of Legionella pneumophila.
Survival and transmission
Microorganisms have evolved life cycles that facilitate their transmission and survival.
Organisms that cause diarrhoea, which are excreted in faeces, spread to other hosts by ingestion (faecal-oral route). More complex examples include organisms with a life-cycle stage inside an insect vector that allows transmission by a biting insect.
Organisms have developed specialist structures and behaviours to favour survival.
Microorganisms propelled from the nose and mouth in a sneeze can remain suspended in the air on droplet nuclei (5 �m). Infection may occur when these are inhaled by another person and are carried to the alveoli. Respiratory infections such as influenza are transmitted in this way, but so are certain infections of other organs (e.g. Neisseria meningitidis).
Food and water contain pathogens that may infect the intestinal tract (e.g. Salmonella). Toxoplasmosis and cysticercosis, which infect organs remote from the gut, are also transmitted by this route.
Insects that feed on blood may transmit a wide range of pathogens: most importantly female anophelene mosquitoes transmit malaria.
Sexual intercourse allows organisms with poor survival ability outside the body to be transmitted. Examples include Neisseria gonorrhoeae, Treponema pallidum and HIV. Transmission of HIV is enhanced by genital ulceration.
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