Infection is an established but uncommon etiology of myositis, and Coxsackie B virus has only been rarely described as a causative agent. We present a case of a 38-year-old male who presented with weakness, myalgias, and testicular pain following two weeks of upper respiratory infection. Laboratory tests revealed an elevated creatine kinase and positive serology for Coxsackie B4 and mumps. This unusual presentation of Coxsackie B myositis and mumps co-infection in a previously healthy young patient illustrates the importance of including infectious etiologies in the differential diagnosis and the potential life-threatening consequences of biased clinical reasoning.