 This is a randomized study medication assigned to participants by chance, double-blind neither physician nor participant knows the identity of the assigned treatment, placebo-controlled, single-center study of JNJ 53718678 in healthy adult participants. Study consists of 3 phases: Screening Day -56 and Study Day -3 prior to the planned date of virus inoculationchallenge on Study Day 0, quarantine includes challenge on Day -1 or -2 and treatment for 7 days, and follow-up Day 15 and 28. Participants will be admitted to the quarantine unit on Study Day -1 or - 2 and will be inoculated intranasal with the RSV-A Memphis 37b virus on Study Day 0. Participants will be evaluated in 3 cohorts: Cohort 1 and Cohort 3 participants will be dosed with JNJ-53718678 or placebo for 7 days and Cohort 2 for up to 7 days Dosing Days 1-x x will be determined based on the results from Cohort 1. Participants will be randomized and JNJ-53718678placebo dosing will be started after RSV presence in nasal wash has been detected by polymerase chain reaction PCR. After completion of Cohort 1, data will be reviewed unblinded and the antiviral activity, safety, pharmacokinetic, clinical symptom and mucus weight data will be evaluated, based on which Cohort 2 will be initiated. Participants safety will be monitored throughout the study. 