Xiaowei Zhuang makes movies of the invisible. Peering into a microscope, she has filmed a single influenza virus infecting a cell. Her studies mark the first time anyone has recorded the stages of this process. Zhuang accomplished this feat by attaching fluorescent molecular tags to the virus; when excited with a laser, the tags emit specific colors of light. She has used the approach to track the behavior of not only individual viruses but even individual molecules, such as strands of RNA, at unprecedented levels of detail. Coming from a traditional physics PhD program, Zhuang very quickly began to lead experiments in single-molecule biophysics as a postdoc in Steven Chus lab at Stanford University. "With total ease, she immersed herself in biological physics and did an astounding amount of seminal work," Chu says. Since establishing her own lab at Harvard, Zhuang has continued to do "landmark experiments at a blistering pace," he adds. Direct observations of individual molecules are essential to really understanding how life works, Zhuang believes. "In the biology world, there are a lot of very small things that are doing critical functions," she says. "There is a lot of interesting dynamic information one can get out of this kind of single-particle approach." In her work on the flu virus, for example, Zhuang discovered that viruses move through the cell in three stages -- one of which is so short that it could only be directly observed with high-speed imaging. "This experiment revealed unprecedented details of virus infection pathways," says Harvard chemist Sunney Xie. Eventually, this in-depth understanding of how viruses work will help researchers find entirely new ways of blocking viral infection, Zhuang says. Indeed, virologists have begun asking to work with Zhuang, hoping to use her methods to study their own pet viruses.