Repeated blood samples turned up important differences in their blood: A group of proteins indicated that a part of the body’s immune system called the complement system remained activated long after it should have returned to normal.

“When you have a viral or bacterial infection, the complement system becomes activated and binds to these viruses and bacteria and then eliminates them,” said Dr. Onur Boyman, a professor of immunology at the University of Zurich in Switzerland and one of the study’s investigators. The system then returns to its resting state, where its regular job is to clear the body of dead cells, he said.

But if the complement system remains in its microbe-fighting state after the viruses and bacteria are eliminated, “it starts damaging healthy cells,” he said.