Rationale: In adults, the pathogens primarily responsible for bacterial meningitis are different from those causing meningitis in neonates, although Listeria is common to both. Although S. pneumoniae is the most common etiology for all adults, Listeria is one of the more common agents, particularly in the immuno-compromised, and in patients more than 50 years old. With altered mental status, encephalitis should always be considered as well. Aseptic (viral) meningitis is quite common but is self-limited and is generally a diagnosis of exclusion.