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 realised 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.

For three years, she was confined to a hospital bungalow on North Brother Island in the East River, but in 1910, she was released after promising to never work as a cook. She took a laundry job but in 1915 was caught cooking in a hospital with an outbreak. She was then confined to North Brother until her death 23 years later.

It has long been known that Salmonella typhi can colonise the gallbladder, living on gallstones largely beyond the reach of antibiotics. [Mallon rebuffed suggestions that she have her gallbladder removed.] The bacteria also invade macrophages, the immune system's "attack cells", which normally engulf and digest invading bacteria.

The new study, led by Denise Monack, a Stanford immunologist, showed in mice that salmonella persist in macrophages that have cycled from an inflammatory state to a non-inflammatory one, and appear to be able to influence the macrophage's metabolism to produce more glucose, which the bacteria feed on.

Monack is also investigating another mystery that emerged during Mallon's imprisonment: On some days her samples were full of salmonella bacteria; on others they had none.