The Gram negative pathogen Capnocytophaga canimorsus is a frequent commensal in the oral cavity of cats and dogs. Although the bacterium is generally considered harmless, infections in humans can occur displaying a broad spectrum of clinical syndromes. This makes a clinical diagnosis difficult. The patient in the present case was 67 years old and presented to the emergency room (ER) with pain in the upper right abdomen and clinical signs of a feverish infection. The only noticeable record in the patient´s medical history was a splenectomy in childhood. The anamnesis revealed that the patient was the owner of two dogs. After a suspected diagnosis of sepsis the patient was transferred to the intensive care unit (ICU), where his medical condition deteriorated rapidly. Despite intensive care measures as well as the fast initialization of a broad-spectrum antibiotic therapy, the patient died 37 h after his presentation in the ER. The search for the causative pathogen turned out to be challenging. Eventually, molecular biological methods assisted in solving the puzzle. It could be demonstrated that the pathogen, found in the patient´s blood, was also present in one of his dogs’ oral cavity.