Meningitis is usually caused by an infection by microorganisms. Most infections are due to viruses, with bacteria, fungi, and protozoa being the next most common causes. It may also result from various non-infectious causes. The term aseptic meningitis refers loosely to all cases of meningitis in which no bacterial infection can be demonstrated. This is usually due to viruses, but it may be due to bacterial infection that has already been partially treated, with disappearance of the bacteria from the meninges, or by infection in a space adjacent to the meninges (e.g. sinusitis). Endocarditis (infection of the heart valves with spread of small clusters of bacteria through the bloodstream) may cause aseptic meningitis. Aseptic meningitis may also result from infection with spirochetes, a type of bacteria that includes Treponema pallidum (the cause of syphilis) and Borrelia burgdorferi (known for causing Lyme disease). Meningitis may be encountered in cerebral malaria (malaria infecting the brain) or amoebic meningitis, meningitis due to infection with amoebae such as Naegleria fowleri, as contracted from freshwater sources.
Bacterial
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). 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. Since the pneumococcal vaccine was introduced, however, rates of pneumococcal meningitis have declined in both children and adults.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. The same pathogens are also more common in those with an impaired immune system. In a small proportion of people, an infection in the head and neck area, such as otitis media or mastoiditis, can lead to meningitis. Recipients of cochlear implants for hearing loss are at an increased risk of pneumococcal meningitis.
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.Recurrent bacterial meningitis may be caused by persisting anatomical defects, either congenital or acquired, or by disorders of the immune system. Anatomical defects allow continuity between the external environment and the nervous system. The most common cause of recurrent meningitis is a skull fracture, particularly fractures that affect the base of the skull or extend towards the sinuses and petrous pyramids. Approximately 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.
