Meningitis is inflammation of the sheaths and membranes (meninges) covering the brain and the spinal cord. Although the most common causes are infection (either bacterial or viral), chemical agents and even tumor cells may cause meningitis. Encephalitis and brain abscess can complicate infective meningitis.

Major bacteria that cause meningitis are streptococcus, Haemophilus influenzae, staphylococcus and meningococcus.

The classical symptoms of meningitis are headache, neck stiffness and photophobia (the trio are called "meningism"). An altered level of consciousness or other neurological deficits may be present depending on the severity of the disease. A lumbar puncture to obtain cerebrospinal fluid is usually indicated to determine the cause and direct appropriate treatment.

Meningitis is a medical emergency with high levels of fatality and the cause is usually a bacteria strain that is highly treatable by antibiotics, patients with suspected meningitis should immediately undergo a lumbar puncture procedure after a CT scan (which will eliminate a possible brain swelling that may cause a brain herniation during lumbar puncture) and broad spectrum antibiotics should be urgently started before the culture studies are completed. If lumbar puncture can not be performed because of brain swelling or a possible concomitant brain abscess, a broad spectrum intravenous antibiotic is started anyway and later it can be replaced with a more specific antibiotic depending on the results of blood culture studies.

Meningitis is usually caused by the infectious disorders of the neighbouring structures (sinuses, mastoid cells of ear etc.). Diagnostic tests should be run to find and treat these possible sources of infection, as well. Convulsions are frequently encountered during the course of meningitis and are treated with appropriate anti-seizure drugs like phenytoin.