Clostridium difficile, a bacterium that causes severe diarrhea, can be difficult to treat with antibiotics. Stefan Hyman/University of Leicester hide caption

itoggle caption Stefan Hyman/University of Leicester

University of Florida researcher Jennifer Stamps administers the peanut butter sniff test to a volunteer. Jesse S. Jones/University of Florida hide caption

itoggle caption Jesse S. Jones/University of Florida

Classical mechanics, represented by Isaac Newton, typically doesn't play nicely with quantum mechanics, represented by Schrodinger's cat. But the 2013 Nobel laureates for chemistry figured out a way to get the two to work together. Courtesy of the Nobel Prize hide caption

itoggle caption Courtesy of the Nobel Prize

Particles of the hepatitis C virus are imaged with an electron microscope. James Cavallini/Science Source hide caption

itoggle caption James Cavallini/Science Source

Hepatitis C patient Nancy Turner shows Kathleen Coleman, a nurse practitioner, where a forearm rash, a side effect of her treatment, has healed. Turner is one of many patients with hepatitis C experimenting with new drugs to beat back the virus. Richard Knox/NPR hide caption

itoggle caption Richard Knox/NPR

Nurse Irena Majola tests Justice Mlambo's blood for HIV at a roadside AIDS testing table in a suburb near Cape Town. Under the "test and treat" strategy, about 45 million South Africans would need to be screened for HIV each year. Rodger Bosch/AFP/Getty Images hide caption

itoggle caption Rodger Bosch/AFP/Getty Images

A cross-sectional X-ray shows what's called a "sunken chest." The bright circle near the bottom is the spine; the gray blob on the right is the heart. Living LLC/Getty Images hide caption

itoggle caption Living LLC/Getty Images

Ruben Bermudez stands in front of a sign that says in Spanish, "To love yourself is to protect yourself." He has struggled to remain eligible for AIDS drug assistance programs since he went on treatment four years ago. Jessica Camille Aguirre/NPR hide caption

itoggle caption Jessica Camille Aguirre/NPR

A PET scan of the brain of a person with Alzheimer's disease. U.S. National Institute on Aging/via Wikimedia Commons hide caption

itoggle caption U.S. National Institute on Aging/via Wikimedia Commons

Brian Lawson rolls a marijuana cigarette at the BC Marijuana Party Headquarters in Vancouver, British Columbia. Vancouver is in the marijuana-friendly corner of Canada. Jae C. Hong/AP hide caption

itoggle caption Jae C. Hong/AP