Final answer:
The reservoir host is what initiates the chain of infection by providing a living environment for the pathogen, which then exits through a portal of exit to another host via a mode of transmission. A susceptible host's vulnerability allows the infection cycle to complete.
Step-by-step explanation:
The link that initiates the chain of infection is the reservoir host. This is where the pathogen lives and multiplies. A reservoir can include humans, animals, or the environment. Transmission of pathogens from infected to non-infected human hosts occurs through different routes. Pathogens must leave the reservoir through a portal of exit, which can be through sneezing, coughing, or bodily fluids, before infecting another host through a portal of entry, such as the skin or mucous membranes. The mode of transmission can be direct or indirect, including contact with contaminated surfaces or through vectors like mosquitoes. Finally, for the infection to take place, there must be a susceptible host with a lowered immunity or a break in the skin that allows the pathogen to enter.