Meningitis is inflammation of the protective membranes covering the brain and spinal cord, known collectively as the meninges.^[1] The inflammation may be caused by infection with viruses, bacteria, or other microorganisms, and less commonly by certain drugs.^[2] Meningitis can be life-threatening because of the inflammation's proximity to the brain and spinal cord; therefore the condition is classified as a medical emergency.^[1][3]

The most common symptoms of meningitis are headache and neck stiffness associated with fever, confusion or altered consciousness, vomiting, and an inability to tolerate light (photophobia) or loud noises (phonophobia). Sometimes, especially in small children, only nonspecific symptoms may be present, such as irritability and drowsiness. If a rash is present, it may indicate a particular cause of meningitis; for instance, meningitis caused by meningococcal bacteria may be accompanied by a characteristic rash.^[1][4]

A lumbar puncture may be used to diagnose or exclude meningitis. This involves inserting a needle into the spinal canal to extract a sample of cerebrospinal fluid (CSF), the fluid that envelops the brain and spinal cord. The CSF is then examined in a medical laboratory.^[3] The usual treatment for meningitis is the prompt application of antibiotics and sometimes antiviral drugs. In some situations, corticosteroid drugs can also be used to prevent complications from overactive inflammation.^[3][4] Meningitis can lead to serious long-term consequences such as deafness, epilepsy, hydrocephalus and cognitive deficits, especially if not treated quickly.^[1][4] Some forms of meningitis (such as those associated with meningococci, Haemophilus influenzae type B, pneumococci or mumps virus infections) may be prevented by immunization.^[1]

The types of bacteria that cause bacterial meningitis vary by age group. In premature babies and newborns up to three months old, common causes are group B streptococci (subtypes III which normally inhabit the vagina and are mainly a cause during the first week of life) and those that normally inhabit the digestive tract such as Escherichia coli (carrying K1 antigen). Listeria monocytogenes (serotype IVb) may affect the newborn and occurs in epidemics. Older children are more commonly affected by Neisseria meningitidis (meningococcus), Streptococcus pneumoniae (serotypes 6, 9, 14, 18 and 23) and those under five by Haemophilus influenzae type B (in countries that do not offer vaccination, see below).^[1][3] In adults, N. meningitidis and S. pneumoniae together cause 80% of all cases of bacterial meningitis, with increased risk of L. monocytogenes in those over 50 years old.^[3][4] Since the pneumococcal vaccine was introduced, however, rates of pneumococcal meningitis have declined in children and adults.^[12]

Recent trauma to the skull gives bacteria in the nasal cavity the potential to enter the meningeal space. Similarly, individuals with a cerebral shunt or related device (such as an extraventricular drain or Ommaya reservoir) are at increased risk of infection through those devices. In these cases, infections with staphylococci are more likely, as well as infections by pseudomonas and other Gram-negative bacilli.^[3] The same pathogens are also more common in those with an impaired immune system.^[1] In a small proportion of people, an infection in the head and neck area, such as otitis media or mastoiditis, can lead to meningitis.^[3] Recipients of cochlear implants for hearing loss are at an increased risk of pneumococcal meningitis.^[13]

Tuberculous meningitis, meningitis due to infection with Mycobacterium tuberculosis, is more common in those from countries where tuberculosis is common, but is also encountered in those with immune problems, such as AIDS.^[14]

Recurrent bacterial meningitis may be caused by persisting anatomical defects, either congenital or acquired, or by disorders of the immune system.^[15] Anatomical defects allow continuity between the external environment and the nervous system. The most common cause of recurrent meningitis is skull fracture,^[15] particularly fractures that affect the base of the skull or extend towards the sinuses and petrous pyramids.^[15] A literature review of 363 reported cases of recurrent meningitis showed that 59% of cases are due to such anatomical abnormalities, 36% due to immune deficiencies (such as complement deficiency, which predisposes especially to recurrent meningococcal meningitis), and 5% due to ongoing infections in areas adjacent to the meninges.^[15]