Endogenous infection

The body’s own endogenous flora can cause infection if the organism gains access to an inappropriate area of the body. This can happen by simple mechanical transfer, for example colonic bacteria entering the female urinary tract. The non-specific host defences may be breached, for example by cutting or scratching the skin and allowing surface commensals to gain access to deeper tissues; this is frequently the aetiology of cellulitis. There may be more serious defects in host immunity owing to disease or chemotherapy, allowing normally harmless skin and bowel flora to produce invasive disease.

Airborne spread

Many respiratory tract pathogens are spread from person to person by aerosol or droplet transmission. Secretions containing the infectious agent are coughed, sneezed, or breathed out, and are then inhaled by a new victim. Some enteric viral infections may also be spread by aerosols of faeces or vomit. Environmental pathogens such as Legionella pneumophila, and zoonoses such as psittacosis, are also acquired by aerosol inhalation, while rabies virus may be inhaled in the dust from bat droppings.

Faeco-oral spread

Transmission of organisms by the faeco-oral route can occur by direct transfer (usually in small children), by contamination of clothing or household items (usually in institutions or conditions of poor hygiene), or most commonly via contaminated food or water. Human and animal faecal pathogens can get into the food supply at any stage. Raw sewage is used as fertilizer in many parts of the world, contaminating growing vegetables and fruit. Poor personal hygiene can result in contamination during production, packaging, preparation or serving of foodstuffs. In the western world, the centralization of food supply and increased processing of food has allowed the potential for relatively minor episodes of contamination to cause widely disseminated outbreaks of food-borne infection.

Table 2-3. Environmental organisms which can cause human infection

Organism	Disease (most common presentations)
Bacteria
Burkholderia pseudomallei	Melioidosis
Burkholderia cepacia	Lung infection in cystic fibrosis
Pseudomonas spp.	Various
Legionella pneumophila	Legionnaires’ disease (pneumonia)
Bacillus cereus	Gastroenteritis
Listeria monocytogenes	Various
Clostridium tetani	Tetanus
Clostridium perfringens	Gangrene, septicaemia
Mycobacteria other than tuberculosis (MOTT)	Pulmonary infections
Fungi
Candida sp.	Local and disseminated infection
Cryptococcus neoformans	Meningitis, pulmonary infection
Histoplasma capsulatum	Pulmonary infection
Coccidioides immitis	Pulmonary infection
Mucor spp.	Mucormycosis (rhinocerebral, cutaneous)
Sporothrix schenkii	Lymphocutaneous sporotrichosis
Blastomyces dermatitidis	Pulmonary infection
Aspergillus fumigatus	Pulmonary infections

Water-borne faeco-oral spread is usually the result of inadequate access to clean water and safe sewage disposal, and is common throughout the developing world. Global coverage for access to clean drinking water is 83% of the world population but global sanitation coverage is currently 58%.

The endogenous skin and bowel commensals can cause disease in the host, either because they have been transferred to an inappropriate site (e.g. bowel coliforms causing urinary tract infection), or because host immunity has been attenuated (e.g. candidiasis in an immunocompromised host). Many infections are acquired from other people, who may be symptomatic themselves or be asymptomatic carriers. Some bacteria, like the meningococcus, are common transient commensals, but cause invasive disease in a small minority of those colonized. Infection with other organisms, such as the hepatitis B virus, can be followed in some cases by an asymptomatic but potentially infectious carrier state.

Zoonoses are infections that can be transmitted from wild or domestic animals to man. Infection can be acquired in a number of ways: direct contact with the animal, ingestion of meat or animal products, contact with animal urine or faeces, aerosol inhalation, via an arthropod vector, or by inoculation of saliva in a bite wound. Many zoonoses can also be transmitted from person to person. Some zoonoses are listed in Table 2.2.

Table 2-2. Zoonotic infections

Disease	Pathogen	Animal reservoir	Mode of transmission
Prions
vCJD	Prion protein	Cattle	Ingestion (CNS tissue)
Viral
Lassa fever	Arenavirus	Multimammate rat	Direct contact
Japanese encephalitis	Flavivirus	Pigs	Mosquito bite
Rabies	Rhabdovirus	Dog and other mammals	Saliva, faeces (bats)
Yellow fever	Flavivirus	Primates	Mosquito bite
Monkey pox	Orthopox virus	Rodents, small mammals	Uncertain
SARS	Coronavirus	Civet cats and small mammals	Droplet
Bacterial
Gastroenteritis	Escherichia coli 0157	Cattle, chickens	Ingestion (meat)
	Salmonella enteritidis and others	Chickens, cattle	Ingestion (meat, eggs)
	Campylobacter jejuni	Various, e.g. chicken	Ingestion (meat, milk, water)
Leptospirosis	Leptospira interrogans	Rodents	Ingestion (urine)
Brucellosis	Brucella abortus Brucella melitensis	Cattle Sheep, goats	Contact; ingestion of milk/cheese
Anthrax	Bacillus anthracis	Cattle, sheep	Contact; ingestion
Lyme disease	Borrelia burgdorferi	Deer	Tick bite
Cat scratch fever	Bartonella henselae	Cats	Flea bite
Plague	Yersinia pestis	Rodents	Flea bite
Typhus	Various Rickettsia spp.	Various	Arthropod bite
Psittacosis (ornithosis)	Chlamydia psittaci	Psittacine and other birds	Aerosol
Others
Toxoplasmosis	Toxoplasma gondii	Cats and other mammals	Ingestion (meat, faeces)
Cryptosporidiosis	Cryptosporidium parvum	Cattle	Ingestion (faeces)
Hydatid disease	Echinococcus granulosus	Dogs	Ingestion (faeces)
Trichinosis	Trichinella spiralis	Pigs, beans	Ingestion (meat)
Toxocariasis	Toxocara canis	Dogs	Ingestion
Cutaneous larva migrans	Ancylostoma caninum	Dogs	Penetration of skin by larvae
Leishmaniasis	Leishmania spp.	Dogs	Ingestion

vCJD, variant Creutzfeldt-Jakob disease; SARS, severe acute respiratory syndrome

Most microorganisms do not have a vertebrate or arthropod host but are free-living in the environment. The vast majority of these environmental organisms are non-pathogenic, but a few can cause human disease (Table 2.3). Person-to-person transmission of these infections is rare. Some parasites may have a stage of their life cycle which is environmental (for example the free-living larval stage of Strongyloides stercoralis and the hookworms) even though the adult worm requires a vertebrate host. Other pathogens can survive for periods in water or soil and may be transmitted from host to host via this route (see below): these should not be confused with true environmental organisms.