Knight (left) and Bucheli take soil samples from beneath one of the decomposing bodies. Katie Hayes Luke for NPR hide caption

itoggle caption Katie Hayes Luke for NPR

The Eagle Mountain Church in Newark, Texas, was linked to at least 21 cases of measles this year, mostly in children. LM Otero/AP hide caption

itoggle caption LM Otero/AP

Drug overdose deaths have more than tripled in the U.S. since 1990. Opioid painkillers like OxyContin are the cause of three-quarters of those deaths. Toby Talbot/Associated Press hide caption

itoggle caption Toby Talbot/Associated Press

Billie Iverson, 86, of Cranston, R.I., recently underwent a transplant of intestinal microbes that likely saved her life. Ryan T. Conaty for NPR hide caption

itoggle caption Ryan T. Conaty for NPR

We may not see them, but we need them. iStockphoto.com hide caption

itoggle caption iStockphoto.com

Could the images common in accounts of near-death experiences be explained by a rush of electrical activity in the brain? Odina/iStockphoto.com hide caption

itoggle caption Odina/iStockphoto.com

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

Streptococcus bacteria, like this strain, can be found in our guts. Janice Haney Carr/CDC Public Health Image Library hide caption

itoggle caption Janice Haney Carr/CDC Public Health Image Library

"Liver buds" grow in petri dishes. The rudimentary organs are about 5 mm wide, or half the height of a classic Lego block. Courtesy of Takanori Takebe/Yokohama City University Graduate School of Medicine hide caption

itoggle caption Courtesy of Takanori Takebe/Yokohama City University Graduate School of Medicine

Bad bug: The bacterium Clostridium difficile kills 14,000 people in the United States each year. Janice Carr/CDC/dapd hide caption

itoggle caption Janice Carr/CDC/dapd

Sarah Murnaghan, on May 30, as she and her parents marked the 100th day of her stay in Children's Hospital of Philadelphia. Murnaghan family/AP hide caption

itoggle caption Murnaghan family/AP