For 50 years, researchers have pursued the idea that cancer cells divide quickly. All the drugs developed so far are aimed at those cells. Such drugs now cure 80 percent of cases of ovarian cancer. But much of that cancer recurs within 2 to 5 years, and when it comes back, it kills many of the patients. The five-year survival rate can be as low as 20 to 30 percent, says Mor. (These figures depend upon when the cancer was first detected and treated.) So what’s going wrong? Mor has identified cells that divide very slowly (“We have movies of them!”), and these cells are not affected by chemotherapy. Indeed, the more chemotherapy a patient receives, the more of these cells she is left with. Some people call these cells cancer stem cells; Mor calls them cancer progenitors. And he has already identified three compounds that promise to block the conversion of these cells into classical cancer cells. (If there’s time, ask Mor about his work on viral infections during pregnancy, which can adversely affect the fetus even if the virus never crosses the placenta.)