Meningitis is the most serious form of Haemophilus influenzae infection, historically causing significant mortality and morbidity, especially among children. Vaccination against H influenzae type b has dramatically decreased the impact of this disease, making recognition more difficult when it occurs. In this article, Dr. Jeffrey A Rumbaugh, from the Division of Neuro-Infectious Disease at Johns Hopkins University, reviews the clinical manifestations and pathophysiology of H influenzae infection, with emphasis on the key features useful for making a timely diagnosis. Recent research has elucidated the factors involved in the inflammatory response to this organism, and new PCR-based techniques for confirming the diagnosis have been developed. The most recent vaccination guidelines and current treatment recommendations are discussed.