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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.
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IC 04-001
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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
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