Typhoid Fever was a pretty nasty disease in the days before antibiotics and vaccines. An outbreak of the disease among the wealthy in New York led to an investigation which found that several affected families had used the same cook: Mary Mallon. She made a great peach ice cream. But Mallon wasn't sick, and certainly didn't understand how she could possibly spread the disease. A symptomless carrier was a new concept for everyone at the time.
Michael Aranda of SciShow tells the story of Mary Mallon, who eventually became known as Typhoid Mary. -via Geeks Are Sexy