"Typhoid Mary was a real person, a woman named Mary Mallon. She made her living as a household cook in New York a hundred years ago. Unfortunately, at some point she was exposed to typhoid fever. In the days before antibiotics, typhoid fever was often fatal. Mary didn’t show any symptoms, but the disease was thriving in her gallbladder.

"From 1900 to 1907, Mary worked for at least eight households which consequently were savaged by typhoid. It’s difficult to know exactly how many people she infected, since she often moved suddenly and never left a forwarding address. In 1907, a doctor noticed that the typhoid outbreaks all had a common element - a heavyset Irish woman named Mary in the household.

"Mary was tracked down and arrested, but she refused to believe the truth. She also refused to learn to wash her hands. She was under house arrest for three years, then released on the condition she never cook again.

"She immediately went back to work as a cook, and spread even more disease for five more years. After she caused a huge outbreak in 1915, she was permanently quarantined on an island for the rest of her life.

"Typhoid Mary remains the poster child for the damage an asymptomatic carrier can do."