Feline Infectious Peritonitis (FIP) remains an important disease in veterinary clinical practice that is often challenging to diagnose. Characterized by a systemic granulomatous/pyogranulomatous vasculitis, FIP is caused by a transformed (mutated) Feline Coronavirus (FCoV) that has acquired the ability to infect macrophages. Macrophage facilitated pathogenesis is augmented by the humoral immune system in a process termed antibody-dependent enhancement. Clinically, FIP presents in two classic clinical syndromes: 1) an exudative or wet form and 2) a non-exudative or dry form. Manifestations represent pathogenic extremes of the FCoV clinical spectrum sharing clinical signs of progressive lethargy, weight loss, anorexia, and fever. There is currently no effective long term treatment for FIP. Prevention strategies are thwarted by mild disease associated with the native virus and the antibody-augmented disease pathomechanism.