Histoplasmosis (also known as "Cave disease," "Darling's disease," "Ohio valley disease," "reticuloendotheliosis," "spelunker's lung" and "caver's disease") is a disease caused by the fungus Histoplasma capsulatum. Symptoms of this infection vary greatly, but the disease affects primarily the lungs. Occasionally, other organs are affected; this is called disseminated histoplasmosis, and it can be fatal if left untreated.