A new Stanford University study solves part of the mystery surrounding Typhoid Mary, the New York Times reports. Scientists have long wondered how Mary Mallon could have infected so many people as a carrier of typhoid fever in the early 1900s, yet appear perfectly healthy for decades. The study offers clues as to how the bacteria that cause the illness, Salmonella typhi, hide out: They invade macrophages, cells in the immune system that typically attack invading bacteria, and then manipulate the metabolism of those macrophages.
Stanford's Scope blog says the bacteria "mess with our immune systems"; the Huffington Post describes the bacteria as 'hacking" the macrophages; Phys.org goes with an analogy, comparing the bacteria to renters who remodel a home. Basically, the bacteria are tough enough that they can survive the first few days of an inflammatory response from the immune system, after which the macrophages go into a "kinder, gentler" anti-inflammatory mode because the body can't handle too much inflammation, and then the bacteria are safe. It's not yet known how the bacteria are able to successfully "flip the switch" on the macrophages, but they also manage to get the macrophages to produce glucose, which the bacteria then feed on—and they can then survive in their new hiding place for decades. The Times notes another mystery: While Mallon's samples were often rife with Salmonella typhi bacteria, at other times they were perfectly clean. (Click for another fascinating health discovery that links our mental health to our gut.)