A 55-year-old man with HIV comes to the emergency department due to 3 weeks of increasing headache, fever, and vomiting. Examination shows low blood pressure and neck stiffness. The patient's condition quickly deteriorates, and he dies in the hospital despite appropriate treatment. Autopsy of the brain shows a diffuse gelatinous exudate covering the base of the brain. Cut sections of the brain show marked ventriculomegaly and frontal lobe infarcts but no intraparenchymal mass lesions.