Fishermen drag a net in Lake Malawi in 2012. About the size of New Jersey, the lake is home to hundreds of fish species and is considered one of the most biologically diverse lakes in the world. Ding Haitao/Xinhua/Landov hide caption

itoggle caption Ding Haitao/Xinhua/Landov

Dr. Lisa Sterman held a Truvada pill at her office in San Francisco in 2012. She prescribed Truvada to patients at high risk for HIV infection even before the Food and Drug Administration approved the medicine explicitly for that purpose. Jeff Chiu/AP hide caption

itoggle caption Jeff Chiu/AP

Contaminated water can spread diseases like cholera and typhoid. A new project aims to provide water filters in the form of an educational book. Soe Than Win/AFP/Getty Images hide caption

itoggle caption Soe Than Win/AFP/Getty Images

Muslim pilgrims wear masks to prevent infection from the Middle East respiratory syndrome in the holy city of Mecca, Saudi Arabia, on Tuesday. Hasan Jamali/AP hide caption

itoggle caption Hasan Jamali/AP

A farmworker in Riyadh, Saudi Arabia, wears a mask to protect against Middle East respiratory syndrome earlier this month. The MERS virus is common in camels. Fayez Nureldine/AFP/Getty Images hide caption

itoggle caption Fayez Nureldine/AFP/Getty Images

Even some euro bank notes may need a good scrubbing. Like dollar bills, these notes are made from cotton and they harbor an array of bacteria. Thomas Leuthard/The Preiser Project/Flickr hide caption

itoggle caption Thomas Leuthard/The Preiser Project/Flickr

An Egyptian Muslim prays during a ritual in Mina, Saudi Arabia, October 2013. Some people wore masks during the hajj pilgrimage last year to protect against the Middle East Respiratory Syndrome. Amr Nabil/AP hide caption

itoggle caption Amr Nabil/AP

Rose Komano, 18 and the mother of three, was the first Ebola patient to overcome the virus in southeastern Guinea, the epicenter of the outbreak. On April 3, she posed at a health clinic in the Gueckedou region. Misha Hussain /Reuters /Landov hide caption

itoggle caption Misha Hussain /Reuters /Landov

Street vendors sell chickens at a market in Phnom Penh, Cambodia, in early 2013. Last year Cambodia reported more cases of H5N1 bird flu than any other country. Mak Remissa/EPA /LANDOV hide caption

itoggle caption Mak Remissa/EPA /LANDOV

Advocates demonstrate in favor of cheaper generic drugs to treat hepatitis C in New Delhi on March 21. The disease is common among people who are HIV positive. Saurabh Das/AP hide caption

itoggle caption Saurabh Das/AP