A Study to Explore the Antiviral Activity, Clinical Outcomes, Safety, Tolerability, and Pharmacokinetics of JNJ-53718678 at Two Dose Levels in Non-Hospitalized Adult Participants Infected With Respiratory Syncytial Virus

The purpose of this study is to explore the antiviral effect of JNJ-53718678 at 2 dose levels (80 milligrams \[mg\] and 500 mg) once daily for 7 days in adults with Respiratory Syncytial Virus (RSV) infection, as measured by RSV viral load in nasal secretions by quantitative reverse transcription polymerase chain reaction (qRT-PCR) assay.

This study will be performed to explore the antiviral activity, clinical outcomes, safety, tolerability, and pharmacokinetics of JNJ-53718678 in adult participants infected with RSV. The study will include both participants who are otherwise healthy (ie, without underlying condition) or who have comorbid conditions (eg, asthma, chronic obstructive pulmonary disease (COPD), cardiovascular disease, other chronic diseases), with the exception of immunocompromised participants, presenting for medical care but not requiring hospitalization. The study will include a screening period (Day -1 to Day 1), a treatment Period (Day 1 to Day 8), and a follow-up period (Day 9 to Day 28). Safety evaluations will include adverse events, laboratory tests, electrocardiogram, vital signs, physical examination, and specific toxicities.

Eligibility Criteria:
Inclusion Criteria:

* Participants must have an acute respiratory illness with signs and symptoms consistent with a viral infection (example, fever, cough, nasal congestion, runny nose, sore throat, myalgia, lethargy, shortness of breath, or wheezing) with onset less than or equal to 5 days from the anticipated time of randomization. Onset of symptoms is defined as the time the participant becomes aware of the first sign and/or symptom consistent with a viral infection
* Participant has been diagnosed with respiratory syncytial virus (RSV) infection using a rapid polymerase chain reaction (PCR) based or rapid-antigen-detection test
* Before randomization, a woman must be not of childbearing potential defined as: Premenarchal, Postmenopausal or Permanently sterile
* A male participant must agree to the use of acceptable contraceptive measures
* With the exception of the RSV-related illness the participant must be medically stable on the basis of physical examination, medical history, vital signs, and electrocardiogram (ECG) performed at screening

Exclusion Criteria:

* Hospitalized participants or participants expected to be hospitalized within 24 hours of screening
* History of or concurrent illness (beyond a comorbid condition) that in the opinion of the investigator, might confound the results of the study or pose an additional risk in administering study drug to the participant or that could prevent, limit, or confound the protocol-specified assessments
* Participants who had major surgery within the 28 days prior to randomization or have planned major surgery through the course of the study
* Participants who are considered by the investigator to be immunocompromised within the past 12 months
* Participant has known or suspected chronic or acute hepatitis B or C infection
* Women who are pregnant or breastfeeding
* Participants with clinically significant abnormal ECG findings (other than QT-interval corrected for heart rate according to Fridericia \[QTcF\] interval greater than \[\>\] 500 millisecond \[ms\]) not consistent with the underlying condition in the study population, as judged by the investigator