Work published in the Jan. 22, 2004, issue of Nature is a significant advance in the understanding of how viruses cause infection, and offers two possible strategies for blocking these infections with antiviral drugs or vaccines. The research involved a major category of viruses known as “enveloped” viruses, so called because of their fatty outer membrane. Class 1 enveloped viruses include influenza and HIV; the new research focuses on class 2 enveloped viruses, responsible for causing dengue fever, West Nile fever, hepatitis C, tick-borne encephalitis, Japanese encephalitis, and other lesser-known diseases. “Many of these are emerging infections,” notes Stephen Harrison, a Howard Hughes Medical Institute investigator and chief of the laboratory of molecular medicine at Children’s Hospital, who was the senior investigator on the study. Led by Yorgo Modis, a structural biologist and postdoctoral fellow in Harrison’s laboratory at Children’s Hospital, the researchers used X-ray crystallography to study a key envelope protein that sits on the membrane of the dengue virus. By aiming an X-ray beam through a crystallized form of the protein, they obtained three-dimensional images, precise down to the atom, showing how a shape change in the protein causes fusion to happen.