infection
The most important source of human infection is other humans. Some agents (e.g. measles) are more transmissible than others. This is measured by the reproductive number that estimates the number of persons one individual infects. An epidemic or outbreak can occur if each infected individual typically transmits the pathogen to more than one additional person i.e., the reproductive number is >1. The route of transmission influences the speed at which diseases can spread as respiratory infections can spread most rapidly. The environment in which transmission occurs is important with infection spreading more rapidly in enclosed places such as prisons. Transmission is more likely when individuals are vulnerable due to, for example, immunocompromising diseases such as HIV. The young too are vulnerable in light of their relatively immature immune systems or poorer hand hygiene, for example, children are very susceptible to tuberculosis early in life if exposed to an adult who is coughing viable bacteria.
Animal pathogens may be spread to humans by direct contact or indirectly through food or materials that have been contaminated by them. Such infections are called zoonoses and are facilitated 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) (see Chapter 63). 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. Materials contaminated with B. anthracis, can transmit anthrax to those who work with them (see Chapter 20).
Survival and transmission
Microorganisms have evolved complex 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.
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 environment.
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 infection.
Airborne/respiratory
Microorganisms propelled from the nose and mouth in a sneeze can remain suspended in the air on droplet nuclei (5 μm). 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).
Faecal–oral
Food and water contain pathogens that may infect the intestinal tract (e.g. Salmonella). Organisms that do not infect the intestine can also be transmitted by this route. Toxoplasmosis and cysticercosis, which infect the brain or muscles, are also transmitted by this route.
Parenteral/transcutaneous
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, for example the blood‐borne viruses HIV and hepatitis B.
Skin organisms (e.g. Staphylococcus epidermidis) can invade the body via indwelling venous cannulae.
Vector‐borne
Insects that feed on blood may transmit a wide range of pathogens: most importantly female anophelene mosquitoes transmit malaria.
Sexual transmission
Sexual intercourse allows organisms that have poor ability to survive outside the body to be transmitted. Examples include Neisseria gonorrhoeae, Treponema pallidum and HIV. Transmission of HIV is enhanced by genital ulceration e.g., caused by syphilis (see Chapters 33 and 27).
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