the.com/chain of infection

means The sequence of links — pathogen, reservoir, exit, transmission, entry, and susceptible host — that must all connect in order for an infection to spread from one being to another.

from Not a word with a romantic root but a public-health model, assembled in epidemiology textbooks and CDC training materials over the 20th century as infection control became a formal science. 'Chain' does the real work here: the metaphor comes from the plain mechanical image of linked rings, where strength lies in the weakest point — snap any single link and the whole chain fails. Infection-control nursing adopted it as a teaching tool precisely because it turns an invisible, frightening process into something you can picture snapping with a gloved hand.