The Chain of Transmission is a visual tool that helps illustrate all the components at play during the acquisition of infectious agents. It is applicable to all environments (e.g. hospital, home and community), but the examples given below will be focused on healthcare settings. Each element in the chain is a topic in and of itself, and the subject of further analysis.
A person who does not have immunity to the infectious agent or is susceptibleas result of being immune-compromised.
Immunity can be acquired through natural infection and immunization, which creates antibodies that protect against the infectious agent. However, several factors can affect your susceptibility as a host, and include:
Age – extremes of age are more likely to be susceptible
Underlying diseases – diabetes and cancer are among those known to affect immunity
Medical interventions – IV lines, catheters, and surgery all create portals of entry; chemotherapy is known to weaken immunity
Poor nutrition and other factors relating to general health
Infectious agents include bacteria, viruses, yeasts, fungi, parasites and prions. Any of these can act as pathogens and cause infection.
Patients harbouring infectious agents are largely responsible for their introduction into healthcare settings. Once there, they can contaminate medical equipment and the environment, and are difficult to eradicate.
A reservoir is a place where an infectious agent can live and reproduce.
Patients are most often the reservoirs for infectious agents within healthcare settings. However, other reservoirs exist, and include food (foodborne pathogens), water and HVAC systems (Legionella).
Refers to the way(s) in which infectious agents travel from reservoirs to hosts.
Modes of Transmission includes Contact, Droplet, Airborne, and various combinations of these routes.
Contact: The most common form of transmission, whereby persons become exposed to pathogens through contact with a person who is infected, or coming into contact with an object that has been contaminated. Also see Direct Contact and Indirect Contact
Droplet: Transmission that occurs as result of large, respiratory droplets containing infectious agents that come into contact with the mucous membranes (e.g. eyes, nose, mouth/throat membranes) of a susceptible host. Droplets can be expelled during coughing, sneezing, laughing and some types of medical procedures (e.g. suctioning), but, as result of their size, typically only travel up to two meters before falling to the ground. Droplets have the potential to land on medical equipment and environmental surfaces, which creates risk for Indirect Contact transmission. Influenza is an example of an infection that spreads through Droplet and Contact transmission.
Airborne: Transmission that occurs as result of tiny, respiratory droplet nuclei particles (1-5 microns in diameter), which enter a susceptible host through inhalation. Droplet nuclei can be expelled during all activities that pass air out of the nose/mouth (e.g. talking, laughing, coughing, etc.) as well as during aerosol-generating medical procedures (e.g. intubation, suctioning, sputum induction, etc.). Once in the air, particles can remain suspended for long periods of time. Tuberculosis is an example of an infection that spreads through Airborne transmission.
