The purpose of the study is to learn whether people who are experiencing an MCD (multicentric Castleman's Disease) flare will improve after taking valganciclovir. MCD is a type of inflammatory disease associated with Human Herpesvirus 8 (HHV-8). Valganciclovir is FDA approved for treating a different type of Human Herpesvirus, but not approved for the treatment of HHV-8. It is therefore considered experimental in this study.