Over the course of human evolution, people with excess stores of fat have been more likely to survive famines, many scientists believe, living on to pass their genes to the next generation.

But these days, obesity is thought to be harmful, leading to chronic inflammation and metabolic disorders that set the stage for heart disease. So what went awry? When did excess fat stop being a protective mechanism that assured survival and instead become a liability?

A provocative new hypothesis suggests that in some people, fat not only stores energy but also revs up the body’s immune system. This subgroup may have enjoyed a survival advantage in the 1800s, when people were plagued by a disease that decimated Europe: tuberculosis.

By some estimates, tuberculosis has killed more than one billion people, eclipsing both the bubonic plague and the Spanish flu.