This study looks at how well the medicine S-337395 works in adults sick with a virus called Respiratory Syncytial Virus (RSV). RSV can make breathing hard, especially for people with other health issues, like lung or heart problems. The study is for people not in the hospital but who are at risk of getting worse. Participants must start the treatment within 72 hours of feeling sick.

Those who can join should be 75 or older, have lung or heart problems, and have RSV symptoms like fever or cough. People shouldn’t join if they're already in the hospital or at risk of being hospitalized soon, have serious kidney problems, or are allergic to S-337395. Also, if they've had certain other infections or treatments recently, they should not participate.

• The study involves taking medicine S-337395 or a placebo.
• Participants need to start within 3 days of RSV symptoms.
• Some health conditions or recent infections can disqualify participation.