The differential diagnosis for a patient presenting with dyspnea, chest pain, altered mental status, and systemic inflammatory response syndrome criteria (fever, tachycardia, and tachypnea) is broad. Based on the history, physical examination, and chest radiograph, the patient almost certainly has sepsis due to pneumonia. The challenge is to determine if pneumonia is the sole cause of his illness. A viral illness followed by secondary bacterial pneumonia could explain his subacute illness followed by acute deterioration but would not fully explain his obtundation. Encephalopathy can occur in septic patients, but severe alterations in consciousness generally occur in the elderly or in patients with hypoperfusion and systemic organ failure. Pneumococcal pneumonia with meningitis is an important consideration; however, the lumbar puncture excludes this possibility. A possible explanation is that the patient has an underlying illness that has been exacerbated by infection. Additional categories of disease that could explain the patient's presentation include cardiovascular disease, toxic ingestions, withdrawal from medications or other agents such as alcohol, and endocrine diseases.