A 22-year-old woman comes to the office because of a 3-day history of cold symptoms and a 1- week history of progressive fatigue. Six weeks ago, she received a kidney transplant from a living, related

A 22-year-old woman comes to the office because of a 3-day history of cold symptoms and a 1- week history of progressive fatigue. Six weeks ago, she received a kidney transplant from a living, related donor. Immediately after the operation, she received monoclonal anti-CD3 therapy. Current medications are azathioprine, cyclosporine, and prednisone. Her temperature is 39°C (102.2°F). Physical examination shows a well-healed surgical scar. Serum studies show that her urea nitrogen and creatinine concentrations have tripled. A diagnosis of allograft rejection is suspected. In addition, this patient's clinical presentation is best explained by an infection with which of the following agents?