This study looks at how a medicine called S-337395 works in adults with a virus called Respiratory Syncytial Virus (RSV). RSV can cause bad cold symptoms. The study is for adults who are not in the hospital but might get very sick from RSV. The medicine is given within 72 hours after symptoms start.

• Length of Study: Participants must start the study within 72 hours of RSV symptoms beginning.
• Eligibility: You need to have symptoms like fever, cough, or trouble breathing, be diagnosed with RSV, and have risk factors like being 75 or older, or having lung or heart problems.
• Exclusions: You can't join if you're in the hospital, have certain allergies, have been treated for RSV recently, or have other serious infections.

This study compares the effects of S-337395 with a placebo, which is a harmless substance used for comparison. Participants should not be severely ill or have received recent RSV vaccinations or treatments before joining. The study is important to see if S-337395 can help stop RSV from getting worse.