Measles is the seventh leading cause of childhood mortality worldwide. Currently 30–40 million measles infections occur each year, leading to 1–2 million deaths. In addition to causing an acute respiratory infection, measles is associated with a profound, transient suppression of cell-mediated immunity. This immunosuppression contributes to the major complications of measles: pneumonia, diarrhea, and other secondary infections. In rare cases, measles can also cause encephalitis and persistent infection of the central nervous system (Griffin and Bellini, 1996). A live-attenuated vaccine controls measles infection in industrialized countries, and measles has been targeted by the World Health Organization for global eradication following the eradication of poliovirus. Nevertheless, measles is endemic in many parts of the world, and recurring outbreaks still occur in industrialized regions for a variety of reasons: First, measles virus (MV) is one of the most contagious infectious agents known, and high levels of vaccine coverage are required to prevent virus circulation in a population. Second, young infants are resistant to vaccination with the attenuated vaccine strain of MV due to maternal antibody interference. Third, while individuals infected with wild-type measles show lifelong immunity, mounting evidence suggests that waning immunity in adult vaccine recipients may allow subclinical infection by MV, providing another source of virus for infants or other susceptible individuals (Clements and Cutts, 1995). Thus, while eradication is a primary goal, frequent reemergences in countries such as the United States (Alaska, 1998), Brazil (1997), Japan (1997–1998), The Netherlands (1999–2000), and Afghanistan (2000) are testament to the challenges that remain for under-