Organ transplantation and major cancer chemotherapy are often associated with prolonged neutropenia (defined as less than 500 polymorphonuclear neutrophils per cubic millimeter), alterations in phagocyte function and lymphopenia due to immunosuppressive agents, and disruption of mucosal defense barriers resulting from mucositis and the presence of indwelling catheters. All these changes can lead to colonization and to invasion by microorganisms — as signaled by fever. More than 30 years ago, the empirical use of broad-spectrum antimicrobial drugs was shown to reduce the frequency of bacteremia and sepsis among such patients. In the 1980s, it was learned that persistent or relapsing fever . . .