Respiratory syncytial virus is a common cause of illness and hospitalization, especially among infants and immunocompromised persons, but there are no accepted antiviral therapies. In this RSV challenge study, GS-5806, a new compound, showed activity against RSV. Respiratory syncytial virus (RSV) infection accounts for substantial morbidity and mortality among infants 1 – 7 and is the most common reason for hospitalization of infants in the United States, 8 with an even greater outpatient burden of disease. Estimates indicate that among children younger than 2 years of age, the annual rate of RSV-related hospitalization is 5.2 per 1000, the rate of emergency department encounters is 32 to 57 per 1000, and the rate of outpatient visits is 66 to 177 per 1000. 2 Among infants younger than 1 year of age, the risk of death from respiratory causes is increased by a . . .