Provided are methods of diagnosing a viral disease such as idiopathic pulmonary fibrosis, Castleman's disease, a lymphoma, a thymoma or a sarcoma in a patient by identifying one or more virus-specific elements such as a nucleic acid or a viral protein or a patient antibody to a virus-specific element, as well as to kits for diagnosing the viral disease in a patient. Further provided are methods of monitoring disease progression and/or the efficacy of therapy by measuring the levels of a virus-specific element in a sample from a patient, and methods of identifying therapeutic agents that show efficacy in reducing levels of virus-specific agents in vitro. Still further provided are methods of treating idiopathic pulmonary fibrosis, a lymphoproliferative disease and cancer, as well as to methods of preventing viral infection, including Herpesvirus saimiri infection.