Typhoid Mary
From Memory Alpha, the free Star Trek reference
"Typhoid Mary" is a human slang term for an individual who is immune to a disease but spreads it to others. It originated in the 20th century from Mary Mallon, a cook who spread typhoid fever but did not show any symptoms herself.
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, the Doctor found that the "Typhoid Mary" was a synthetic pathogen introduced into a nearby Borg vinculum by Species 6339. (VOY: "Infinite Regress")
