The overall purpose of this research is to develop and use a blood test to better understand how quickly the viral drug ganciclovir works to clear infection with the CMV virus (Cytomegalovirus) when it occurs. This test will potentially let doctors know early in the course of therapy when a virus is not responding well to the therapy and could therefore be resistant to the drug. The target population of this study will be primarily kidney and lung transplant patients with CMV detected in the blood, although other patients may also be included if they meet criteria. The study will be divided into two phases. Phase I will evaluate a small number of exploratory patients initiating ganciclovir therapy and will require frequent blood sampling to obtain detailed information regarding the kinetic response of the virus to therapy. This information will be analyzed to help guide decisions regarding the number and frequency of blood samples needed in the larger phase II portion of the study. Strains will be characterized using phenotypic and genotypic methods to determine the presence or absence of mutations potentially responsible for the resistance.