Cytotoxic T Lymphocytes in Treating Patients With Malignancies With BK and/or JC Virus

This study is testing specially engineered immune cells (called cytotoxic T lymphocytes, or CTLs) as a treatment for patients who have serious viral infections caused by BK virus or JC virus — viruses that can cause severe kidney damage or brain infection in people with weakened immune systems, certain cancers, or organ transplants. **You may be eligible if...** - You are 2 years of age or older - You have a weakened immune system (such as from a transplant, HIV/AIDS, or cancer treatment), OR you have JC virus brain infection (called PML), OR you have Merkel cell cancer - Blood or urine tests confirm a BK or JC virus infection, or kidney inflammation due to BK virus - Your current antiviral medications have not worked - You are able to reduce steroid medications to a low dose - Women of childbearing age must have a negative pregnancy test and agree to use contraception **You may NOT be eligible if...** - You are currently on high-dose steroids - You received certain powerful immune-suppressing treatments very recently - You have other active, uncontrolled infections (beyond HIV) - You have severe acute graft-versus-host disease (grades II to IV) after a bone marrow transplant Talk to your doctor to see if this trial is right for you.