Final answer:
The proper terms for each step in the chain of infection are portal of exit, portal of entry, and susceptible host.
Step-by-step explanation:
The proper term for the step in the chain of infection where an infectious agent leaves its reservoir is portal of exit. This is the way by which the pathogen exits the body of the infected individual. The pathogen finds its way into the body of another person through a portal of entry, which is a way for the pathogen to enter the body. Finally, a susceptible host refers to a person with reduced defenses against a specific agent, making them more likely to become infected.