Every medical phenomenon has both a mechanistic explanation and an evolutionary explanation. Veterinarians are accustomed to dealing with the mechanistic, the "what" or the "how", of various disease conditions, and applying treatment accordingly. Darwinian medicine is a field that addresses the evolutionary explanation, the "why" for various medical conditions. This review focuses on these Darwinian explanations and is divided into 4 main categories--host defenses, virulence, genetic conflict, and incomplete adaptation to a changing environment. Each of these areas is reviewed, with examples of evolutionary reasons for disease conditions. Consideration of adaptationist reasons for many of these disease phenomena should make veterinarians better clinicians, educators, and researchers.