Cytomegalovirus (CMV) infection is the most common opportunistic infection in lung transplantation leading to direct and indirect effects that can result in life threatening complications. The risk of CMV infection is highest when the recipient of the transplant has never been in contact with CMV (negative immunity) and the donor had previous contact with CMV (positive immunity). This is called CMV mismatch. For these lung transplant patients 6 to 12 months of prophylaxis with an antiviral called Valganciclovir is recommended. This antiviral can cause side effects like bone marrow toxicity and decrease in immune cells which can result in temporarily having to stop the treatment. Starting and stopping the prophylaxis may result in the CMV becoming resistant to the medication. While taking the prophylaxis it is possible to have a breakthrough of the CMV, this is often due to the development of resistance to the antiviral. The purpose of this study is to learn more about the rate of CMV breakthrough while on prophylaxis after lung transplantation in patients who are CMV mismatch. The investigators will also look at the rates of negative side effects caused by antiviral prophylaxis in this population.