Sources of infection
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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.
- Following bowel perforation Enterobacteriaceae and non-sporing
anaerobes such as Bacteroides fragilis can invade the peritoneal
cavity causing peritonitis and septicaemia.
- Inhalation of stomach contents may cause pneumonia, possibly
followed by a lung abscess.
- Staphylococcus aureus that are normally found in the nose may
cause wound infection if inoculated into a surgical wound.
- Neutropenic patients are especially prone to infection from
organisms normally held in check by the body's defences.
- Surgery and intravenous cannulation allow skin organisms to
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
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
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.
- Bacterial spores have a tough coat and low metabolic rate that
enable them to survive for many years.
- Helminth eggs have a tough shell adapted for survival in the
- The host can aid survival: when the infecting organism is able
to persist for a long time in the host, this acts as a reservoir of
Microorganisms propelled from the nose and mouth in a sneeze
can remain suspended in the air on droplet nuclei (5 Ám). 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
- Leptospira, Treponema and Schistosoma have evolved specific
mechanisms enabling them to invade intact skin.
- Injections, whether medical or for taking illegal drugs, and blood
transfusions bypass the skin, allowing the transmission of a range
organisms including the blood-borne viruses HIV and hepatitis B.
- Skin organisms (e.g. Staphylococcus epidermidis) can invade the
body via indwelling venous cannulae.
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.