“The Cancer Detectives” tracks the fascinating and frustrating backstory of the Pap smear, a cervical cancer screening test that’s now routine but was once anything but. A century ago, cervical cancer was a major killer of women. Early detection was impossible, and sexual stigma and shame kept women from discussing it. It’s not surprising that when an immigrant physician from Greece discovered a way to detect changes in cervical cells, his breakthrough was largely disregarded by the scientific community.