The chain of infection is a way of gathering the information needed to interrupt or prevent an epidemic. Each of the links in the chain must be favorable to the organism for the epidemic to continue. Breaking any link in the chain can disrupt the epidemic. The link that is most effective to the target will depend on the organism.
INFECTIOUS AGENT (PATHOGEN)
Any disease-causing germ that includes viruses, fungi, bacteria and parasites.
RESERVOIR (SOURCE)
Where the pathogen lives or comes from. A reservoir can be an infected person. Food, water, an animal and dirt can also be reservoirs.
PORTAL OF EXIT (EXIT)
The way the pathogen leaves its reservoir. For example, suppose the reservoir is a person, and the pathogen is a cold virus. The pathogen can exit the person s nose or mouth through a sneeze or a cough.
MODE OF TRANSMISSION (TRANSMISSION)
How the pathogen moves from the reservoir to the susceptible host. In the sneeze example above, the pathogen is carried in the sneeze droplets. Other examples of transmission modes are sexual contact, animal bite and needle stick.
PORTAL OF ENTRY (ENTRY)
Where the pathogen enters the body of the susceptible host. For example, if a person sneeze then droplets could land on the susceptible hosts eyes, nose or mouth. These areas would be the portal of entry. Broken skin is another common portal of entry for pathogens.
SUSCEPTIBLE HOSTS (HOSTS)
Almost everyone is a susceptible host when it comes to cold viruses. In other cases, some people are susceptible to a particular infection and others are not. Factors such as weak immunity or lack of vaccination can make a host susceptible to disease.
