Development of an infection is dependent upon an uninterrupted process, referred to as – chain of infection. This process is dependent upon the following elements:
This chain of infection can be broken by infection control measures implemented by health care workers. These measures either terminate the threat or block the mechanism leading to the next link in the chain. An infection may be stopped at any point in the chain of infection.
A reservoir is any person, animal, arthropod, plant, soil, or substance (or combination of these) in which an infectious agent normally lives and multiplies, on which it depends primarily for survival, and where it reproduces itself in such manner that it can be transmitted to a susceptible host. There can be Animate reservoirs include people, insects, birds, and other animals. Inanimate reservoirs include soil, water, food, feces, intravenous fluid and equipment.
The pathogen then leaves the reservoir via the portal of exit. This is the site from where micro-organisms leave one host to enter another host and cause disease or infection. For example, a micro-organism may leave the reservoir through the nose or mouth when someone sneezes or coughs, or in feces. Ports of exit include: Upper respiratory tract, Gastrointestinal tract, Blood, Urogenital tract, Skin and mucous membranes.
Once a pathogen has exited the reservoir, it needs a mode of transmission to the host through a portal of entry. Transmission can be by direct or indirect contact or through airborne transmission.
Direct contact is person-to-person transmission of pathogens through touching, biting, kissing, or sexual intercourse. Microorganisms can also be expelled from the body by coughing, sneezing or talking. The organisms travel in droplets over less than 1 meter in distance and are inhaled by a susceptible host.
Indirect contact includes both vehicle-borne and vector-borne contact. A vehicle is an inanimate go-between, an intermediary between the portal of exit from the reservoir and the portal of entry to the host. Inanimate objects such as handkerchiefs and tissues, soiled laundry, and surgical instruments and dressings are common vehicles that can transmit infection.
The infectious agents then enter the body through a portal of entry. After an infectious agent gets inside the body it has to multiply in order to cause the disease. In some hosts, infection leads to the disease developing, but in others it does not. Individuals who are likely to develop a communicable disease after exposure are called susceptible hosts. This is due to a low level of immunity within the more susceptible individuals. Immunity refers to the resistance of an individual to communicable diseases, because their white blood cells and antibodies (defensive proteins) are able to fight the infectious agents successfully. Low levels of immunity could be due to:
The chain of infection can be broken and infection avoided or disrupted by breaking any one of the links within the chain. For instance, healthcare workers may avoid the transmission of pathogens by using standard precautions. Immunization against common pathogens such as influenza, measles and pertussis will thwart may viral agents. Additional examples follow: