It is a serious infection of the meninges (the membranes covering the brain). Meningitis may be due to a viral or bacterial infection. Bacterial meningitis is usually far more dangerous. Causes of bacterial meningitis include both Gram positive and Gram negative bacteria. The most common causes are Pneumococcus (Streptococcus pneumoniae), Haemophilus (Haemophilus influenzae) and Meningococcus (Neisseria meningitidis).
Bacteria usually reach the meninges via the blood stream. Rarely, infection is by direct spread, e.g. from mastoiditis. Tuberculosis also causes bacterial meningitis. Fungal meningitis may be seen in children with AIDS.