A Study to Assess the Pharmacokinetics, Safety, and Tolerability of Multiple Doses of Orally Administered JNJ-53718678 in Infants Hospitalized With Respiratory Syncytial Virus (RSV) Infection

The purpose of this study is to evaluate pharmacokinetics, safety, tolerability, antiviral activity, and impact on the clinical course of Respiratory Syncytial Virus (RSV) infection after multiple oral doses of JNJ-53718678 at different doses and/or dosing regimens in infants (greater than \[\>\] 1 month to less than or equal to \[\<=\] 24 months of age) who are hospitalized with RSV infection.

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.

Eligibility Criteria:
Inclusion Criteria:

* Participant has presented at the hospital for suspected Respiratory Syncytial Virus (RSV) infection within 72 hours prior to Screening completion
* Participant has been hospitalized for this suspected RSV infection
* Participant has been diagnosed with RSV infection using a polymerase chain reaction (PCR)-based assay, preferably commercially available locally
* Participant was born after a normal term pregnancy (greater than or equal to 37 weeks and 0 days)
* A legally acceptable representative of the participant must sign an Informed consent form (ICF) indicating that he or she understands the purpose of and procedures required for the study, are willing for their child to participate in the study, are willing for their child to remain in the hospital for the first 3 days of dosing (even if not clinically indicated), and are willing/able to adhere to the prohibitions and restrictions specified in the protocol and study procedures

Exclusion Criteria:

* Participant who had major surgery within the 28 days prior to randomization or planned major surgery through the course of the study
* Participant has major congenital anomalies or known cytogenetic disorders
* Participant has known or suspected immunodeficiency, such as known human immunodeficiency virus (HIV) infection
* Participant has known or suspected hepatitis B or C infection
* Participant is upon current admission initially hospitalized in the Intensive care unit (ICU) and/or in need of invasive endotracheal mechanical ventilation