Population 1: Potential Baloxavir treatment group (CHOP transplant subjects 5 years & up) - Willing CHOP male or female kidney, heart, liver or lung single or multiple transplant recipients aged 5 years or older as per FDA guidelines. - Willing to regularly wear a smartwatch and take an at-home positive respiratory virus (RV) panel which will include a diagnoses of Influenza A or B. - Have an antigen positive diagnoses of Influenza A or B (a PCR-based positive clinical diagnoses of Influenza A or B may be requested in "alarm positive plus antigen positive but asymptomatic" cases). - Can be included if their treating physician prescribe prophylactic treatment of Baloxavir if the subject has been exposed to Influenza. - If Baloxavir is prescribed the study subject should be treated within 48 hours of symptom onset (regardless of the alarming time). Population 2: Potential Baloxavir treatment group (CHOP waitlisted subjects 5 years & up) - Willing waitlisted CHOP kidney, heart, liver or lung single or multiple transplant recipients aged 5 years or older, which are anticipated to have a transplant in the next 12 months. - All other inclusion criteria listed in 3.3.1. Population 3: Potential Baloxavir treatment group (non-transplanted household members) - Non-transplanted household member of a CHOP transplant recipient or waitlisted patient - Be at least 5 years of age. - Willing to regularly wear a smartwatch and take an at-home positive respiratory virus (RV) panel for diagnoses of Influenza A or B. - Have a Antigen-based positive diagnoses of Influenza A or B Population 4: Non-Baloxavir treatment subjects - CHOP transplant recipients and all other non-transplanted household members who are 2-4 years of age. - Subjects 5 years and up who are Influenza positive but whom do not receive Baloxavir treatment

Population 1: - Any allergy to Baloxavir (although they can remain in the study as an influenza case or control without Baloxavir treatment, or if they have been treated with a different medication for influenza) or a recommendation from the study physicians'/transplant pharmacist(s) not to take Baloxavir. - Subjects weighing < 40 kg - If the subject is unable or unwilling to consent. - If the subject is younger than 5 years of age. - If the subject requires mechanical ventilation at time of enrollment. - If the subject is pregnant or breast feeding at the time of early infection alerting. - If the subject is taking a prohibited medication. These include Influenza antiviral drugs with the exception of oseltamivir and baloxavir (such as peramivir, laninamivir, zanamivir, rimantadine, umifenovir or amantadine). - Unwilling or unable to comply with the study requirements. Population 2: All exclusion criteria listed for Population 1 Population 3: - Subjects weighing < 40 kg - A household transplant recipient is not participating in the study - Any allergy to Baloxavir (although they will remain in the study as an influenza case/control without treatment) - A recommendation from the study physicians'/transplant pharmacist not to take Baloxavir - If the subject is unable or unwilling to consent. - If the subject is younger than 5 years of age. - If the subject is pregnant at screening. - If the subject is taking a prohibited medication. These include Influenza antiviral drugs with the exception of oseltamivir and baloxavir (such as peramivir, laninamivir, zanamivir, rimantadine, umifenovir or amantadine). - Unwilling or unable to comply with the study requirements.