Cytomegalovirus (CMV) is a common cause of illness in patients who have undergone a transplant. Serious infections due to CMV can affect many parts of the body including the lungs, the gut, and the liver. Since transplant recipients are at risk for CMV or have evidence of infection with CMV, they are given an antiviral drug (usually ganciclovir or valganciclovir). Despite this, there are a chance that CMV infection may cause problems in the future. The purpose of this study is to assess how well patients'immune systems responds to the CMV virus, so that in the future it may be possible to predict which patients are at highest risk of CMV.