A growing number of scientists is trying to help people reach a goal humans have been chasing throughout history: staying active and vital in old age. They are part of the skyrocketing scientific field of cellular senescence, which is built upon the idea that cells eventually stop dividing. The body removes most of them, but others linger like zombies, not quite dead and harming normal cells nearby. They accumulate in older bodies, and mounting evidence links this to an array of age-related conditions like dementia, cardiovascular disease and osteoporosis. Scientists wonder: Can the zombie cell buildup be stopped through things like drugs or exercise?