• Participants with symptoms suggestive of influenza based on investigator's judgement with diagnosis confirmed by a positive local influenza test within 24 hours before full study screening

• Participants with a negative severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) test within 48 hours before full study screening

• Time interval between onset of influenza symptoms and the pre-dose examinations at screening is 48 hours or less

⁃ \[A\] IP:

• Eligible to take part in Part A

• Lives in a household with a HHC willing to be recruited as full household contact

⁃ \[B\] HHCs:

⁃ \- Each HHC living in the home of an IP at the time of IP treatment must be screened for partial or full-study HHC eligibility

⁃ \[C\] Partial HHC:

• Starts screening within 1 calendar day after IP treatment

• Negative influenza and SARS-CoV-2 test at screening after IP's treatment with baloxavir marboxil

• HHC lives in a household where 1 or more HHCs meet the full-study HHC requirements AND at least 1 full-study HHC has not received the influenza vaccine within 6 months prior to screening

• HHC lives with other HHCs (if applicable) who fulfill all the Partial household contact criteria

⁃ \[D\] Full-study HHC:

• Fulfills the Partial household contact criteria

• Agrees to participate in the full study

• Will reside in the IP's house for at least 12 of the next 15 days and will be present for scheduled study visits

• No influenza symptoms within 7 days prior to screening

• Does not have a moderate or worse active infections OR infections requiring systemic or otherwise internally administered or otherwise internally administered antibiotic/antiviral/antifungal therapy