Inclusion Criteria:  Adult patients: Signed informed consent by any patient capable of giving consent, or, where the patient is not capable of giving consent, by his or her legal/authorized representative Age greater than or equal to 18 years at the time of signing the Informed Consent Form/Assent Form Ability to comply with the study protocol, in the investigator's judgment Have received allogeneic bone marrow transplant Tested positive for influenza infection after the onset of symptoms using a polymerase chain reaction (PCR)-based diagnostic assay. Presence of (a) fever (≥38.0 °C per tympanic or rectal thermometer; ≥ 37.5 °C per axillary, oral or forehead/temporal thermometer) or (b) any influenza symptoms (cough, sore throat, nasal congestion, headache, feverishness or chills, muscle or joint pain, fatigue). The time interval between the diagnosis of influenza and the pre-dose examinations is 48 hours or less. For women of childbearing potential: Agreement to remain abstinent (refrain from heterosexual intercourse): Women must remain abstinent or use contraceptive methods with a failure rate of < 1% per year during the treatment period and for 28 days after the last dose of study treatment. Hormonal contraceptive methods must be supplemented by a barrier method. A woman is considered to be of childbearing potential if she is postmenarcheal, has not reached a postmenopausal state greater than or equal to 12 continuous months of amenorrhea with no identified cause other than menopause), and has not undergone surgical sterilization (removal of ovaries and/or uterus). Examples of contraceptive methods with a failure rate of < 1% per year include bilateral tubal ligation, male sterilization, hormonal contraceptives that inhibit ovulation, hormone-releasing intrauterine devices, and copper intrauterine devices. The reliability of sexual abstinence should be evaluated in relation to the duration of the clinical trial and the preferred and usual lifestyle of the patient. Periodic abstinence (e.g., calendar, ovulation, symptothermal, or postovulation methods) and withdrawal are not acceptable methods of contraception.  Exclusion Criteria:  Patients who have received more than 48 hours of antiviral treatment for the current influenza infection prior to screening Patients who have received Baloxavir for the current influenza infection Known contraindication to neuraminidase inhibitors Patients weighing < 40 kg Patients unable to swallow tablets Patients with known severe renal impairment (estimated glomerular filtration rate < 30 mL/min/1.73 m2) or receiving continuous renal replacement therapy, hemodialysis, peritoneal dialysis Patients with any of the following laboratory abnormalities detected within 24 hours prior to or during screening (according to local laboratory reference ranges: ALT or AST level > 5 times the upper limit of normal (ULN) OR ALT or AST > 3 times the ULN and total bilirubin level > 2 times the ULN Pregnant or breastfeeding, or positive pregnancy test in a predose examination, or intending to become pregnant during the study or within 28 days after the last dose of study treatment Exposure to an investigational drug within 5 half-lives or 30 days (whichever is longer) of randomization Any serious medical condition or abnormality in clinical laboratory tests that, in the investigator's judgment, precludes the patient's safe participation in and completion of the study Known hypersensitivity to baloxavir marboxil or the drug product excipients Known COVID-19 coinfection Unwilling to undergo nasopharyngeal (NP) swabs as per study schedule