The chlamydia bacteria can cause pelvic inflammatory disease and fertility problems, but women often don't know they're infected. David M. Phillips/Science Source hide caption

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Dr. George Papanicolaou discovered that it was possible to detect cancer by inspecting cervical cells. The Pap smear, the cervical cancer screening test, is named after him. American Cancer Society/AP hide caption

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

toggle 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

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