Additional Study Information: Are you undergoing a stem cell transplant? You may be at high risk for infections after the transplant. This study is looking at the safety and effectiveness of an investigational medication for those who have undergone a stem cell transplant. Recovery after a stem cell transplant can already be challenging without the added worry of life-threatening viral reinfections. It may be worth exploring the possibilities and peace of mind a clinical trial can offer. This is a Phase 2/3 study to evaluate the efficacy and safety of ALVR105 (also known as Viralym-M and formerly known as ALVR-105) for the prevention of clinically significant adenovirus (AdV), BK virus(BKV), cytomegalovirus (CMV), Epstein-Barr virus (EBV), human herpesvirus 6 (HHV-6), and JohnCunningham virus (JCV) infections and/or disease in patients at high risk for these viruses following allogeneic hematopoietic cell transplant (HCT). Patients that qualify and choose to enroll in the study may be reimbursed for study-related travel expenses. Participation for this study is divided into three periods: 1. Screening: up to 10 weeks, 2. Treatment: 14 weeks, 3. Follow-up: 12 weeks