Genetic testing indicated the presence of methylenetetrahydrofolate reductase (MTHFR) (C677T) CT type and plasminogen activator inhibitor-1 (PAI-1) (4G/5G) 4G/5G type, while pleural fluid sequencing confirmed Epstein–Barr virus (EBV)/cytomegalovirus (CMV) infection, leading to a diagnosis of viral myocarditis. The gene discussed is MTHFR; the disease is viral myocarditis.