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INFECTION CONTROL MANUAL PRINCIPLES OF TRANSMISSION OF MICROORGANISMS In hospital epidemiology, routes of transmission of infectious agents have been classified as: contact, airborne, common vehicle and vector borne. The route of transmission is the way in which microorganisms reach a susceptible host. 1. Contact Transmission: includes direct contact, indirect contact, and droplet transmission. 1.1. Direct contact transmission occurs when microorganisms are transferred by direct physical contact between an infected or colonized individual and a susceptible host (body surface to body surface). 1.2. Indirect contact transmission occurs when microorganisms are transferred to a susceptible host by an intermediate object such as contaminated instruments/medical devices, or other inanimate objects in the patient’s environment. 1.3. Droplet transmission occurs when large particles called droplets are generated from a person’s respiratory tract during such activities as speaking, coughing, or sneezing. Droplets are propelled a short distance (less than 2 meters) through the air and deposited on the mucous membranes of another person. The infected person and susceptible host need to be relatively close to each other. Large droplets do not remain suspended in the air. Some microorganisms that are expelled in droplets, especially respiratory viruses, remain viable after the droplets settle on objects in the immediate environment of the patient. Viruses can survive long enough on surfaces to be picked up on the hands of patients or personnel. Microorganisms may be transmitted by direct deposition of infectious droplets onto the nasal mucosa or conjunctiva (droplet), or by inoculation of these membranes by contaminated hands (contact). 2. Airborne Transmission - Refers to the spread of microorganisms from the source patient over varying distances in small particle aerosols. Microorganisms are contained in droplet nuclei or in dust particles containing skin squames and other debris that remain suspended in the air for long periods of time. 2.1. The microorganisms are widely dispersed by air currents and inhaled by susceptible hosts. Control of airflow, by means of special ventilation systems, is required for control of this type of transmission. Transmission of Microorganisms, Principles of IC 04-001 Page 1 of 2 Infection Control Manual 3. Common Vehicle Transmission - Occurs when a single contaminated source, such as: food, water, medications, intravenous fluid, or equipment serves to transmit infection to multiple hosts. Control is by maintenance of appropriate standards in the preparation of food, medications and the decontamination of equipment. 4. Vector-borne Transmission - Refers to spread of infection by insects to susceptible hosts. It is a less frequent method of transmission in the health care setting of most industrialized nations. REFERENCES Association for Professionals in Infection Control and Epidemiology Inc. (2009). APIC Text of Infection Control and Epidemiology (third edition), Washington, DC. Health Canada (July, 1999). Canada Communicable Disease Report (Volume 25S4). Routine Practices and Additional Precautions for Preventing the Transmission of Infection in Health Care. Transmission of Microorganisms, Principles of IC 04-001 Page 2 of 2