Meningitis is usually caused by infection by microorganisms. Most cases are due to infection with viruses, followed by bacteria, fungi, and parasites. Viruses that can cause meningitis include various enterovirus subtypes, herpes simplex virus 2 (and less commonly HSV-1), varicella zoster virus (known for causing chickenpox and shingles), mumps virus and HIV.
The types of bacteria that cause bacterial meningitis vary by age group. In premature babies and newborn up to three months, common bacteria are group B streptococcus (subtype III)–especially in the first week of life and bacteria 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 adults, N. meningitidis and S. pneumoniae together cause 80% of all cases of meningitis, with increased risk of L. monocytogenes in those over 50. In trauma, neurosurgery, or other contact between the skin and the meninges, staphylococci are more likely, as well as infections with pseudomonas and related Gram-negative bacilli. The same pathogens are also more common in those with an impaired immune system. 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.
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). Fungal meningitis, e.g. due to Cryptococcus neoformans, is typically seen in people with immune deficiency such as AIDS. Amoebic meningitis, meningitis due to infection with amoebae such as Naegleria fowleri, is contracted from freshwater sources.
Recurrent bacterial meningitis may be caused by 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 skull fracture, particularly fractures that affect the base of the brain or extend towards the sinuses and petrous pyramids. A literature review of 363 reported cases of recurrent meningitis showed that 59% of cases occur due to 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.
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