This April 2019 microscope photo provided by the Niedernhofer Lab of the Institute on the Biology of Aging and Metabolism at the University of Minnesota shows shows two senescent human fibroblast cells, above, next to normal ones in Minneapolis, Minn. "Zombie" cells start out normal but then encounter a stress, like damage to their DNA or viral infection. At that point, a cell can choose to die or become a zombie, which means they stop going through their normal cycle of activities. The problem is that they then start secreting substances that can harm nearby normal cells. (Matthew Yousefzadeh, Mariah Witt/University of Minnesota via AP)