When a patient with a history of recent IVDU presents with a complaint of fever, the mind of the provider should immediately focus on the numerous possible infectious complications that can arise.  Infectious endocarditis can lead to septic emboli spread to any organ system.  Pneumonia can result from aspiration or septic embolization. Cellulitis/abscess can obviously result from local injection.  But what about when a source of fever is not readily identifiable? When cultures are negative and the patient’s symptoms have resolved, what could have been the cause of their febrile illness?