"Typhoid Mary" was a nickname coined during the mid-20th century to describe a Human female who was responsible for spreading the deadly typhoid pathogen while remaining immune herself.
In 2269, Doctor Leonard McCoy compared Doctor Sevrin – who carried, but was immune to Synthococcus novae – to "Typhoid Mary". (TOS: "The Way to Eden")
When Seven of Nine began exhibiting multiple personalities in 2375 because of a dysfunctional Borg vinculum infected with a virus, The Doctor found that in this case, the "Typhoid Mary" was Species 6339 that carried a synthetic pathogen within their bodies; once they were assimilated by the Borg, the virus spread to the vinculum, which in turn caused havoc, leading to dysfunction and eventually the destruction of the Borg cube's drones. (VOY: "Infinite Regress")
The script for DS9: "The Dogs of War" described Odo as the "typhoid Mary" of the morphogenic virus he infected the Female Changeling with. This was the original concept, but then the DS9 writing team realised that it was more apt for Odo to get sick, unlike Typhoid Mary. (Star Trek: Deep Space Nine Companion, p. 691