Ali Maow Maalin said he avoided getting the smallpox vaccine as a young man because he was afraid of needles. He didn't want others to make the same mistake with polio. Courtesy of the World Health Organization hide caption

itoggle caption Courtesy of the World Health Organization

A harmful trio (from left): a deer tick, lone star tick and dog tick. Getty Images hide caption

itoggle caption Getty Images

It's busy down there: a gut bacterium splits into two, becoming two new cells. Centre For Infections/Science Photo Library/Corbis hide caption

itoggle caption Centre For Infections/Science Photo Library/Corbis

A Yemeni child receives a polio vaccine in the capital city of Sanaa. The Yemen government launched an immunization campaign last month in response to the polio outbreak in neighboring Somalia. Mohammed Huwais/AFP/Getty Images hide caption

itoggle caption Mohammed Huwais/AFP/Getty Images

Children with tuberculosis sleep outside at Springfield House Open Air School in London in 1932. Like sanatoriums, these schools offered TB sufferers a place to receive the top treatment of the day: fresh air and sunshine. Fox Photos/Getty Images hide caption

itoggle caption Fox Photos/Getty Images

A Doctors Without Borders support counselor waits for MDR-TB patients at a clinic in Nukus, Uzbekistan. Courtesy of Misha Friedman hide caption

itoggle caption Courtesy of Misha Friedman

People living in affluent neighborhoods in north central Dallas were most likely to get infected in 2012. Those neighborhoods were also hit hardest in the 2006 outbreak. JAMA hide caption

itoggle caption JAMA

Health workers vaccinate a boy against polio at a May immunization drive in Mogadishu, Somalia. Farah Abdi Warsameh/AP hide caption

itoggle caption Farah Abdi Warsameh/AP

Saudi men walk to the King Fahad hospital in the city of Hofuf on Sunday. In eastern Saudi Arabia, where outbreaks of the MERS virus have been concentrated, people have resumed their habits of shaking hands and kissing. Fayez Nureldine/AFP/Getty Images hide caption

itoggle caption Fayez Nureldine/AFP/Getty Images

Spinal MRIs similar to these found infections that many patients hadn't realized they had. Stefano Raffini/iStockphoto.com hide caption

itoggle caption Stefano Raffini/iStockphoto.com

A woodcut from the 1800s, Healing the Lepers, depicts the common tableau of Jesus healing a leper as his disciples look on. Images from the History of Medicine hide caption

itoggle caption Images from the History of Medicine

Boys at the L'Ecole Les Freres Clement elementary school in Jacmel, Haiti, line up to take deworming pills that protect against elephantiasis. Maggie Steber for The Washington Post/Getty Images hide caption

itoggle caption Maggie Steber for The Washington Post/Getty Images

The Centers for Disease Control and Prevention says doctors should prescribe Truvada, a once-a-day pill for HIV, to help prevent infections in IV drug users. Jeff Chiu/AP hide caption

itoggle caption Jeff Chiu/AP