Scientists are one step closer to explaining how Typhoid Mary could have infected dozens of New Yorkers over a 12-year career as a cook, killing at least three of them, without having ever been sick herself.

A new study by scientists at Stanford University’s medical school, published this month in the journal Cell Host & Microbe, sheds more light on how Salmonella typhi, the bacteria that cause typhoid fever, hide in the body.

Typhoid outbreaks are now rare in wealthy countries with flush toilets, but in poor ones where raw sewage flows in gutters, there may be up to 20 million annual cases causing 200,000 deaths.

Mary Mallon, known as Typhoid Mary, is the disease’s most famous asymptomatic carrier. Health officials investigating outbreaks in homes in New York from 1900 to 1907 realized she had worked in many of them. She was arrested and forced to produce stool and urine samples, which showed she was healthy but shedding huge amounts of bacteria. She said she rarely washed her hands. Although her cooked food was probably safe, she was also famous for her peach ice cream.