Understanding the Spread of Infection

Infections are illnesses that are caused by germs (microorganisms) such as bacteria, viruses, fungi, and parasites. For an individual to get an infection, a process involving six components must occur. This process is called the chain of transmission. The goal of infection prevention and control practices is to break a LINK in the chain of transmission to prevent the transfer of microorganisms.

Chain of Transmission Components (links):

Infectious Agent

Any microorganism that can cause infection. This can be a bacterium, virus, parasite, fungus or prion.

Reservoir

Any place that an infectious agent can live and reproduce. This can include people, contaminated food or water, insects, or animals.

Portals of Exit

The way the infectious agent leaves the reservoir. This can occur through the mouth and nose (e.g., coughing or sneezing), gastrointestinal tract excretions (e.g., vomiting or diarrhea), blood, or drainage from wounds.

Modes of Transmission

The way the infectious agent travels. This can be through:
Direct contact from one person to another (i.e., touching), or indirect contact (i.e., from one person to an object or surface and then to another person).
Droplets when a person coughs or sneezes.
The air (airborne) when extremely small particles are released from one person and then inhaled by another person.

Portals of Entry

The way an infectious agent enters a susceptible host. The individual may breathe it in, touch their eyes, nose or mouth, or eat or drink contaminated food or water. The infectious agent may also enter through breaks in the skin.

Susceptible Host

Any person who is at risk of developing an infection. The individual risk will depend on the infectious agent and personal factors such as age, immune system function, and vaccination history, among others.

Breaking a Link in the Chain of Transmission

Breaking any link in the chain of transmission will stop the transmission of the infectious agent and stop the spread of infection.

Ways to stop the spread of infection include:

Frequent hand washing
Receive your recommended vaccinations
Stay home when you are sick
Cover your coughs and sneezes
Clean and disinfect frequently touched surfaces and objects
Proper food preparation and storage
Water treatment
Treatment with antimicrobials, when appropriate

References

Ottawa Public Health. (2024). Chain of Infection. Retrieved from Ottawa Public Health website.
Provincial Infectious Disease Advisory Committee (2012). Routine Practices and Additional Precautions in All Health Care Settings. Toronto, Canada: Queen's Printer for Ontario
Public Health Ontario. (2020). IPAC Core Competencies - Risk Assessment and Chain of Transmission.

Contact the Communicable Disease Control (CDC) program at 705-474-1400 or toll free at 1-800-563-2808, ext. 5229, or by email to cdc@healthunit.ca for more information.

Last updated: Sept 2025, by CDC