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.

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%.

Vector-borne disease

Many tropical infections, including malaria, are spread from person to person or from animal to person by an arthropod vector. Vector-borne diseases are also found in temperate climates, but are relatively uncommon. In most cases part of the parasite life cycle takes place within the body of the arthropod, and each parasite species requires a specific vector. Simple mechanical transfer of infective organisms from one host to another can occur, but is rare.

Direct person-to-person spread

Organisms can be passed on directly in a number of ways. Sexually transmitted infections are dealt with on page 117. Skin infections such as ringworm, and ectoparasites such as scabies and head lice, can be spread by simple skin-to-skin contact. Other organisms are passed on by blood- (or occasionally other body fluid) to-blood transmission. In some cases such as HIV and hepatitis B virus this is the only route: in others such as malaria and Chagas' disease it is an unusual alternative to the normal arthropod vector. Blood-to-blood transmission can occur during sexual contact, from mother to infant peripartum, between intravenous drug users sharing any part of their injecting equipment, when infected medical equipment is reused, if contaminated blood or blood products are transfused, or in any sporting or accidental contact when blood is spilled.

Direct inoculation

Infection can occur when pathogenic organisms breach the normal mechanical defences by direct inoculation. Some of the circumstances in which this can occur are covered under endogenous infection and blood-to-blood transmission above. Some environmental organisms may be inoculated by accident: this is a common mode of transmission of tetanus and certain fungal infections. Rabies virus may be inoculated by the bite of an infected animal.

Consumption of infected material

Although many food-related zoonotic infections are due to contamination of food with animal faeces (and are thus, strictly speaking, faeco-oral), several diseases are transmitted directly in animal products. These include some strains of salmonella (eggs, chicken meat), brucellosis (unpasteurized milk), and the prion diseases kuru and vCJD (neural tissue).