A Surveillance Study of Susceptibility to Baloxavir Marboxil in Pediatric Participants With Influenza and Transmission of Influenza to Household Contacts

Inclusion Criteria (Part A): * 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 start of pre-dose examinations at screening is 48 hours or less Inclusion Criteria (Part B): \[A\] IP: * Eligible to take part in Part A * Lives in a household with a HHC willing to be recruited as full HHC \[B\] 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 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 HHC" criteria \[D\] Full-study HHC: * Fulfills the "partial HHC" criteria * Agrees to partic…