Infections of the central nervous system are potentially treatable emergencies. Imaging plays a central role in the early diagnosis of brain infections and in the detection of related complications, in patients presenting to the ED. While many organisms share non-specific and overlapping manifestations, others have characteristic imaging findings that may allow a prompt confident diagnosis. We describe the manifestations and imaging appearance of the most common acquired acute pyogenic, viral, mycobacterial, fungal, and parasitic infections which may lead to a neurological emergency. Acute opportunistic infections in patients with acquired immunodeficiency syndrome (AIDS) will also be described.