| a carrier or transmitter of anything undesirable, harmful, or catastrophic. |
n] 
| Mary (“Typhoid Mary”), 1869?–1938, U.S. cook, born in Ireland: known immune carrier of typhoid fever who infected many with the disease, institutionalized in 1914. |
Ty·phoid Mary  (tī'foid') n. A person from whom something undesirable or deadly spreads to those nearby. [From the nickname of Mary Mallon.] |
A person likely to cause a disaster; from Mary Mallen, an Irish woman in the United States who was discovered to be a carrier of typhoid fever.
A cook who carried typhoid fever and passed it on to many people in and around New York City in the early twentieth century.
Note: The term is often applied to the carrier of a contagious disease, or, more generally, to anyone who brings bad luck: “The last three insurance companies I had policies with folded. I feel like Typhoid Mary.”
typhoid Mary
A carrier or spreader of misfortune, as in I swear he's a typhoid Mary; everything at the office has gone wrong since he was hired. This expression alludes to a real person, Mary Manson, who died in 1938. An Irish-born servant, she transmitted typhoid fever to others and was referred to as "typhoid Mary" from the early 1900s. The term was broadened to other carriers of calamity in the mid-1900s.