The links of the chain of infection include a pathogen or infectious agent, reservoir, portal of exit, method of transmission and portal of entry, according to the Missouri Department of Health and Senior Services. The final link is the susceptible host. Breaking one or more of the links stops an infectious disease from spreading.
The habitat in which an infectious agent lives, develops and multiplies is a reservoir, which can be an animal, a human and the environment, according to the U.S. Centers for Disease Control and Prevention. A reservoir does not have to be the source from which an infection transfers to a host. Soil is a reservoir for botulism, but contaminated canned foods are the means of transferring the infected spores. Human reservoirs transmit infections such as strep, mumps, measles and sexually transmitted diseases. Carriers can have no symptoms and transmit diseases unknowingly.
A pathogen leaves the reservoir or host through a portal of exit, usually the pathogen's location, explains the CDC. Portals of exit include skin lesions and cuts, urine, feces, blood, and a mother's placenta to her fetus. Methods of transmission can be direct, such as human-to-human touching, or indirect, such as airborne dust or droplets. Inanimate objects, such as surgical equipment or handkerchiefs, and vectors, such as ticks and mosquitoes, can transmit infections. The portal of entry gives the pathogen access into the susceptible host's tissue, where the pathogen multiplies; the portals of entry and exit may be identical, such as blood in hepatitis B infections.