The past few years have witnessed an explosive increase in our collective knowledge of the biology of the human immunodeficiency virus (HIV). Researchers have acquired new understanding of the virus's biochemistry, molecular biology, pathogenesis, genetics, and immunobiology. Resulting therapeutic advances have significantly prolonged the lives of thousands. Yet, the need to develop better therapies is ever more acute and--given the virus's continued spread through the human population--the need for an effective vaccine is urgent.

These goals can be accomplished only through the...