JNJ-53718678 is an investigational respiratory syncytial virus (RSV) specific fusion inhibitor and is under development for the treatment of RSV infection, which results in an upper and/or lower respiratory tract illness. The primary hypothesis of this study is that JNJ-53718678 has antiviral activity against RSV (that is, results in a decrease in RSV nasal viral load from immediately prior to first dose of study drug until Day 5). This will be assessed by a positive dose-response relationship of JNJ-53718678 compared to placebo. Besides the RSV nasal viral load through day 5, other timepoints will also be evaluated as well as other nasal viral load related parameters. In addition, the evolution of signs and symptoms of RSV disease will be evaluated. Participants' safety will be monitored throughout the study by evaluating the occurrence and severity of adverse events and by laboratory and electrocardiogram measurements. Study participants will be identified when they are hospitalized or expected to be hospitalized within 24 hours after presentation to the hospital (Cohort 1) or present for medical care as outpatients (Cohort 2) with symptoms of an acute respiratory illness supporting a diagnosis of RSV infection. Eligible participants will be randomized 1:1:1 to receive either a low or a high dose of JNJ 53718678 or placebo and will be receiving study treatment for 7 days. They will be followed up for 3 weeks after the last dose. The total study duration for each participant will be approximately 29 days.