Two cervical cancer cells divide in this image from a scanning electron microscope. Steve Gschmeissner/Getty Images/Science Photo Libra hide caption

itoggle caption Steve Gschmeissner/Getty Images/Science Photo Libra

Cells gathered during a Pap test. Those on the left are normal, and those on the right are infected with human papillomavirus. Ed Uthman/Wikimedia Commons hide caption

itoggle caption Ed Uthman/Wikimedia Commons