This is a Phase 1b, randomized (study medication assigned to participants by chance), partially double-blind (neither physician nor participant knows the identity of the assigned treatment), placebo-controlled, multicenter, multiple ascending dose study of JNJ 53718678 in infants (greater than [>] 1 month to less than or equal to [<=] 24 months of age) who are hospitalized with RSV infection. The duration of study will be approximately 4 weeks for each participant excluding screening period. In Part 1 of study, minimum total number of 42 participants will be divided in 3 cohorts: Age group 1 (Cohorts 1a-1e) (greater than or equal to [>=] 6 months and less than or equal to [<=] 24 months of age), Age group 2 (Cohorts 2a-2e)(>=3 months and less than [<] 6 months of age) and Age group 3 (Cohorts 3a-3e) (greater than [>] 1 month and <3 months of age). Each age group will consist of a minimum of 3 cohorts with the possibility to add 2 more per age group (Cohorts a through e) in which different doses and/or dosing regimens will be evaluated. Each cohort will consist of 5 participants (4 participants receiving JNJ-53718678 and 1 participant receiving placebo for 7 days), except for the first cohort of each age group which will contain only 4 participants (4 participants receiving JNJ 53718678). In Part 2 of the study, all age groups will be included in a single cohort, Cohort f, in which the selected dose regimen determined during Part 1 of the study will be used for each of the 3 age groups. A minimum of approximately 18 (12 participants receiving JNJ 53718678 and 6 participants receiving placebo) and a maximum of 24 participants (16 participants receiving JNJ 53718678 and 8 participants receiving placebo) will be included in this part of the study. Pharmacokinetics and safety of JNJ-53718678 will be evaluated primarily. Participants' safety will be monitored throughout the study.