Inclusion Criteria:  Age ≥18 years on day of signing informed consent Listed for lung transplantation (single or double) due to a diagnosis of IPF or receipt of a lung transplant (single or double) for IPF in the 72 hours prior to enrollment Have a documented positive serostatus for CMV (CMV IgG seropositive, R+) Have a documented negative serostatus for CMV (CMV IgG seronegative, R-) and anticipate receiving or having received a lung allograft from a CMV IgG positive donor, D+). Only participants who are R+ or who are CMV D+/R- will receive intervention. Participants who are CMV D-/R- will be considered screen failures Able to travel to UPMC for routine post-transplant visits for a minimum of 15 months after transplantation Able to provide informed consent Be willing to use a contraceptive method while receiving LET and for at least 90 days following last dose of LET  Exclusion Criteria:  Receipt of a previous solid organ transplant or hematopoietic stem cell transplant Multi-organ transplant recipient, i.e., heart-lung or lung-liver HIV seropositive HCV antibody or HCV RNA positive Donor HCV NAT positive Anticipated need for use of ganciclovir, valganciclovir, foscarnet, or cidofovir at the time of transplant Known or suspected hypersensitivity to LET or acyclovir CrCl < 10 ml/min or dialysis on day of transplant Child-Pugh Class C severe hepatic insufficiency Pregnancy or expected to conceive while on LET and through at least 90 days following cessation of LET