Patient description: A 17 year old boy complains of vomiting, non-bloody diarrhea, abdominal pain, fever, chills and loss of appetite for the past 3 days. He ate a salad at a restaurant prior to his diarrhea onset. Physical exam was remarkable for pallor, jaundice, and diffuse abdominal tenderness. Lab results were as follows: Hemoglobin: 9.7 g/dL Platelet: 110,000 /cu.mm Creatinine: 3.6 mg/dL blood urea nitrogen (BUN): 73 mg/dL direct bilirubin: 2.4 mg/dL lactate dehydrogenase (LDH): 881 IU/L (normal: 110-265 IU/L) Peripheral blood smear showed a moderate number of schistocytes and helmet cells. Shiga-like toxin-producing E. coli (STEC) stx1/stx2 were found in stools. He has no other underlying disease and he is not on any medications.

Clinical trial eligibility criteria: eligible ages (years): 12.0-64.0,  Influenza Patients who are able to understand the study and comply with all study procedures, and willing to provide written informed consent/assent prior to the predose examinations appropriately. As for adolescent patients, informed consent/assent of voluntary participation should be obtained in accordance with local requirements 2. Male or female patients aged ≥ 12 to ≤ 64 years at the time of signing the informed consent/assent form. 3. Patients with a diagnosis of influenza virus infection confirmed by all of the following: 1. Fever ≥ 38ºC (axillary) in the predose examinations or > 4 hours after dosing of antipyretics if they were taken 2. At least one of the following general systemic symptoms associated with influenza are present with a severity of moderate or greater Headache Feverishness or chills Muscle or joint pain Fatigue 3. At least one of the following respiratory symptoms associated with influenza are present with a severity of moderate or greater Cough Sore throat Nasal congestion 4. The time interval between the onset of symptoms and the predose examinations is 48 hours or less. The onset of symptoms is defined as either: 1. Time of the first increase in body temperature (an increase of at least 1ºC from normal body temperature) 2. Time when the patient experiences at least one general or respiratory symptom 5. Women of childbearing potential who agree to use a highly effective method of contraception for 3 months after the first dose of study drug  Patients with severe influenza virus infection requiring inpatient treatment. 2. Patients aged ≥ 20 years with known allergy to oseltamivir (Tamiflu®). 3. Patients with any of the following risk factors 1. Women who are pregnant or within 2 weeks post-partum 2. Residents of long-term care facilities (eg, welfare facilities for the elderly, nursing homes) 3. Chronic respiratory diseases including bronchial asthma 4. Neurological and neurodevelopmental disorders including disorders of the brain, spinal cord, peripheral nerve, and muscle (eg, cerebral palsy, epilepsy [seizure disorders], stroke, intellectual disability, moderate to severe developmental delay, muscular dystrophy, or spinal cord injury) 5. Heart disease (such as congenital heart disease, congestive heart failure, or coronary artery disease), excluding hypertension without any other heart-related symptoms) 6. American Indians and Alaskan natives 7. Blood disorders (such as sickle cell disease) 8. Endocrine disorders (including diabetes mellitus) 9. Kidney disorders 10. Liver disorders 11. Metabolic disorders 12. Compromised immune system (including patients receiving immunosuppressant therapy, or those with cancer or human immunodeficiency virus [HIV] infection) 13. Morbid obesity (body mass index [BMI] ≥ 40) 4. Patients unable to swallow tablets or capsules. 5. Patients who have previously received Baloxavir Marboxil. 6. Patients weighing < 40 kg 7. Patients who have been exposed to an investigational drug within 30 days prior to the predose examinations. 8. Women who are breastfeeding or have a positive pregnancy test in the predose examinations. The following female patients who have documentation of either a or b below do not need to undergo a pregnancy test in the predose examinations: 1. Postmenopausal (defined as cessation of regular menstrual periods for 2 years or more and confirmed by a follicle-stimulating hormone test) women 2. Women who are surgically sterile by hysterectomy, bilateral oophorectomy, or tubal ligation 9. Patients with concurrent infections requiring systemic antimicrobial and/or antiviral therapy at the predose examinations. 10. Patients who have received peramivir, laninamivir, oseltamivir, zanamivir, rimantadine, umifenovir, or amantadine within 30 days prior to the predose examinations. 11. Patients who have received an investigational monoclonal antibody for a viral disease in the last year. 12. Patients with severe underlying diseases. 13. Patients with known creatinine clearance ≤ 60 mL/min. 14. Patients who, in the opinion of the investigator, would be unlikely to comply with required study visits, self-assessments, and interventions