I've heard this term thrown around in reference to someone who irresponsibly passes their illness to friends, coworkers and family members, BUT did you ever look up the origin of this story? It's a strange one...
http://en.wikipedia.org/wiki/Typhoid_Mary She worked as a cook in the New York City area between 1900 and 1907. She had been working in a house in Mamaroneck, New York for less than two weeks when the residents came down with typhoid. She moved to Manhattan in 1901 and members of the family for whom she worked developed fevers and diarrhea and the laundress died. She then went to work for a lawyer until seven of the eight household members developed typhoid. Mary spent months helping to care for the people she made sick, but her care further spread the disease through the household. In 1906, she took a position in Long Island. Within two weeks, six out of eleven family members were hospitalized with typhoid. She changed employment again and three more households were infected.
People catch typhoid fever after ingesting water or food which has been contaminated during handling by a human carrier. The human carrier is usually a healthy person who has survived a previous episode of typhoid fever but in whom the typhoid bacteria have been able to survive without causing further symptoms. Carriers continue to excrete the bacteria in their excrement and urine. It takes vigorous scrubbing and sanitation to remove the bacteria from the hands. Though Mary washed her hands every time she used the bathroom, she still infected people.
When typhoid researcher George Soper approached Mallon with the news she was possibly spreading typhoid, she adamantly rejected his request for urine and stool samples. Soper left and later published his findings in the June 15, 1906 issue of the Journal of the American Medical Association. On his next contact with her, he brought a doctor with him, but was again turned away. Mallon's denials that she was a carrier were based in part on the diagnosis of a reputable chemist who had found she was not harboring the bacteria. It is possible she was in temporary remission when tested. Moreover, when Soper first told her she was a carrier, the concept that a person could spread disease and remain healthy was not well known. Finally, George Soper may have been somewhat tactless in his dealings with her. During a later encounter in the hospital, he told Mary he would write a book about her and give her all the royalties. She got up and locked herself in the bathroom until he left.