Meningitis is inflammation of the 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 pneumoniae, 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 (CSF) is usually indicated to determine the cause and direct appropriate treatment.