In acute bacterial meningitis, age is an impoant factor relating the etiologic agent and the prognosis. Among adults (greater than 15 years), Streptococcus pneumoniae is responsible for 30-50% of meningitis infections, rather than Neisseria meningitidis, which is responsible for 10-35% of them. Among children (one month to five years), Haemophilus influenzae is the most common cause of bacterial meningitis (40-60%), followed by Neisseria meningitidis (25-40%) and Streptococcus pneumoniae (10-20%). Among neonates (less than one month), meningitis caused by gram-negative bacilli (50-60%) is the most common bacterial cause, followed by Streptococcus group A and group B (20-40%), and Listeria monocytogenes (2-10%). The fatality rate is high among the elderly due to S. pneumoniae; therefore, they should be given pneumococcal vaccine as a preventative. Ref: Ray C.G., Ryan K.J. (2010). Chapter 30. Neisseria. In C.G. Ray, K.J. Ryan (Eds),Sherris Medical Microbiology, 5e.