Any mechanism by which a susceptible human host is exposed to an infectious or parasitic agent. These mechanism are:- 1. Direct transmission Direct and essentially immediate transfer of infectious agents (other than from an arthropod in which the organism has undergone essential multiplication or development) to a receptive portal of entry by which infection of humans may take place. This may be by touching, as in kissing, sexual intercourse or biting (direct contact); or by the direct projection of droplet spray onto the conjunctivae, or onto the mucous membranes of the nose or mouth during sneezing, coughing, spitting or talking (usually not possible over a distance greater than 3 ft) (droplet spread); or, as in the systemic mycoses, by direct exposure of susceptible tissue to soil, compost or decaying vegetable matter that contains the agent and where it normally leads a saprophytic existence. 2. Indirect transmission (a) Vehicle-borne Contaminated materials or inanimate objects such as toys, handkerchiefs, soiled clothes, bedding (fomites), surgical instruments or dressing (indirect contact); water, food, milk, biological products including serum and plasma, or anysubstance serving as an intermediate means by which an infectious agent is transported and introduced into a susceptible host through a suitable portal of entry. The agent may or may not have multiplied or developed in or on the vehicle before being introduced into man. (2) Vector-borne (i) Mechanical:- Includes simple mechanical carriage by a crawling or flying insect through soiling of its feet or proboscis, or by passage of organisms through its gastrointestinal tract. This does not require multiplication or development of the organism. (ii) Biological:- Propagation (multiplication), cyclic development, or a combination of them (cyclopropagation) is required before the arthropod can transmit the infective form of the agent to man. An incubation period (extrinsic) is required following infection before the arthropod becomes infective. Transmission may be by saliva during biting, or by regurgitation or deposition on the skin of agents capable of penetrating subsequently through the bite wound or through an area of trauma following scratching or biting. This is transmitted by an infected invertebrate host and must be differentiated for epidemiological purposes from simple mechanical carriage by a vector in the role of a vehicle. An arthropod in either role is termed a vector. (c) Air-borne The dissemination of microbial aerosols with carriage to suitable portal of entry, usually the respiratorytract. Microbial aerosols are suspensions in air of particles consisting partially or wholly of microorganisms. Particles in the 1 to 5 micron range are quite easily drawn into the lungs and retained there. They may remain suspended in the air for long periods of time, some retaining and others losing infectivity of virulence. Not considering as airborne are droplets and other large particles, which promptlysettle out; the following are airborne, their mode of transmission indirect: (i) Droplet nuclei: Usually the small residues which result from evaporation of droplets emitted by an infected host. Droplet nuclei also may be created purposely by a variety of atomising devices, or accidentally, in microbiology laboratories or in abattoirs, rendering plants, autopsy rooms, etc. They usuallyremain suspended in the air for long periods of time. (ii) Dust: The small particles of widely varying size which may arise from contaminated floors, clothes, beddings, other articles; or from soil (usually fungus spores separated from dry soil by wind or mechanical stirring). Note: Air conditioning and similar air circulating systems may play a significant role in air-borne transmission (e.g. Legionnaire’s disease).
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