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

To fight antibiotic-resistant staph germs like these, a study suggests disinfecting the skin of all intensive care patients. Janice Carr/CDC hide caption

itoggle caption Janice Carr/CDC

Fungi (cyan) surround a human hair within the skin. A study in the journal Nature shows the population of fungi on human skin is more diverse that previously thought. Alex Valm, Ph.D. hide caption

itoggle caption Alex Valm, Ph.D.

A scientist removes the nucleus from a human egg using a pipette. This is the first step to making personalized embryonic stem cells. Courtesy of OHSU Photos hide caption

itoggle caption Courtesy of OHSU Photos

Sucking may be one of the most beneficial ways to clean a baby's dirty pacifier, a study found iStockphoto.com hide caption

itoggle caption iStockphoto.com

A window into dreams may now be opening. Silver Screen Collection/Getty Images hide caption

itoggle caption Silver Screen Collection/Getty Images