This term uses a metaphor to create a visual interpretation; each link depends on the last and the next, in order to continue. Break one link and there is no longer a connection. An example might be:
The 1st link is the Infectious Agent or pathogen, let’s use MRSA
The 2nd link is the Reservoir or house the pathogen lives in. In this case it’s living in the nares (nose) of a nurse but not causing any clinical signs of infection
The 3rd link is the Portal of Exit, or the path the pathogen takes out of the reservoir (the nurse rubs her nose, now the pathogen is on her hands)
The 4th link is the Mode of Transmission or the way the pathogen is passed. This can be direct, indirect, ingesting or inhaling. In this case the Mode of Transmission is direct, because the nurse rubs her nose, the MRSA is on her hands and she then touches the patient’s skin
The 5th link is the new host, her patient
The 6th link is the portal of entry. The nurse does not take time to wash her hands and transmits the MRSA to her patient’s skin while taking her blood pressure.
The patient is a susceptible host because her immune status is impaired by her chronic medical conditions and diabetes. Her arm itches, she scratches it, the MRSA she picked up from the nurse is carried directly into her open skin (portal of entry) and she develops an MRSA wound infection.
In reviewing the situation, we see one of the links could have been broken at any time, to prevent this infection. The only barrier needed was simple, effective hand washing or the use of an alcohol sanitizer, which could have prevented transmission at every link.
Here is a good link to a visual interpretation of Breaking the Chain of Infection: Break-the-Chain-of-Infection
Courtesy of APIC.ORG