<unk>  is typically caused by an infection with 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<unk>  refers to cases of<unk>  in which no bacterial infection can be demonstrated. This type of<unk>  is usually caused by viruses but it may be due to bacterial infection that has already been partially treated, when bacteria disappear from the meninges, or pathogens infect a space adjacent to the meninges (e.g. sinusitis). Endocarditis (an infection of the heart valves which spreads small clusters of bacteria through the bloodstream) may cause aseptic<unk> . Aseptic<unk>  may also result from infection with spirochetes, a group of bacteria that includes Treponema pallidum (the cause of syphilis) and Borrelia burgdorferi (known for causing Lyme disease).<unk>  may be encountered in cerebral malaria (malaria infecting the brain) or amoebic<unk> ,<unk>  due to infection with amoebae such as Naegleria fowleri, contracted from freshwater sources. Bacterial Streptococcus pneumoniae—a causative bacteria of<unk>  (illustration). The types of bacteria that cause bacterial<unk>  vary according to the infected individual's 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 bacteria that normally inhabit the digestive tract such as Escherichia coli (carrying the K1 antigen). Listeria monocytogenes (serotype IVb) can be contracted when consuming improperly prepared food such as dairy products, produce and deli meats, and may cause<unk>  in the newborn. * Older children are more commonly affected by Neisseria<unk> is (meningococcus) and 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, Neisseria<unk> is and Streptococcus pneumoniae together cause 80% of bacterial<unk>  cases. Risk of infection with Listeria monocytogenes is increased in persons over 50 years old. The introduction of pneumococcal vaccine has lowered rates of pneumococcal<unk>  in both children and adults. Recent skull trauma potentially allows nasal cavity bacteria to enter the meningeal space. Similarly, devices in the brain and meninges, such as cerebral shunts, extraventricular drains or Ommaya reservoirs, carry an increased risk of<unk> . In these cases, the persons are more likely to be infected with Staphylococci, Pseudomonas, and other Gram-negative bacteria. These pathogens are also associated with<unk>  in people with an impaired immune system. An infection in the head and neck area, such as otitis media or mastoiditis, can lead to<unk>  in a small proportion of people. Recipients of cochlear implants for hearing loss are more at risk for pneumococcal<unk> . Tuberculous<unk> , which is<unk>  caused by Mycobacterium tuberculosis, is more common in people from countries in which tuberculosis is endemic, but is also encountered in persons with immune problems, such as AIDS. Recurrent bacterial<unk>  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<unk>  is a skull fracture, particularly fractures that affect the base of the skull or extend towards the sinuses and petrous pyramids. Approximately 59% of recurrent<unk>  cases are due to such anatomical abnormalities, 36% are due to immune deficiencies (such as complement deficiency, which predisposes especially to recurrent meningococcal<unk> , and 5% are due to ongoing infections in areas adjacent to the meninges. Viral Viruses that cause<unk>  include enteroviruses, herpes simplex virus (generally type 2, which produces most genital sores; less commonly type 1), varicella zoster virus (known for causing chickenpox and shingles), mumps virus, HIV, LCMV, Arboviruses (acquired from a mosquito or other insect), and the Influenza virus. Mollaret's<unk>  is a chronic recurrent form of herpes<unk> ; it is thought to be caused by herpes simplex virus type 2. Fungal There are a number of risk factors for fungal<unk> , including the use of immunosuppressants (such as after organ transplantation), HIV/AIDS, and the loss of immunity associated with aging. It is uncommon in those with a normal immune system but has occurred with medication contamination. Symptom onset is typically more gradual, with headaches and fever being present for at least a couple of weeks before diagnosis. The most common fungal<unk>  is cryptococcal<unk>  due to Cryptococcus neoformans. In Africa, cryptococcal<unk>  is now the most common cause of<unk>  in multiple studies, and it accounts for 20–25% of AIDS-related deaths in Africa. Other less common fungal pathogens which can cause<unk>  include: Coccidioides immitis, Histoplasma capsulatum, Blastomyces dermatitidis, and Candida species. Parasitic A parasitic cause is often assumed when there is a predominance of eosinophils (a type of white blood cell) in the CSF. The most common parasites implicated are Angiostrongylus cantonensis, Gnathostoma spinigerum, Schistosoma, as well as the conditions cysticercosis, toxocariasis, baylisascariasis, paragonimiasis, and a number of rarer infections and noninfective conditions. Non-infectious<unk>  may occur as the result of several non-infectious causes: spread of cancer to the meninges (malignant or neoplastic<unk>  and certain drugs (mainly non-steroidal anti-inflammatory drugs, antibiotics and intravenous immunoglobulins). It may also be caused by several inflammatory conditions, such as sarcoidosis (which is then called neurosarcoidosis), connective tissue disorders such as systemic lupus erythematosus, and certain forms of vasculitis (inflammatory conditions of the blood vessel wall), such as Behçet's disease. Epidermoid cysts and dermoid cysts may cause<unk>  by releasing irritant matter into the subarachnoid space. Rarely, migraine may cause<unk> , but this diagnosis is usually only made when other causes have been eliminated.
Meningitis
Lyme disease
Cerebritis
Naegleriasis
Encephalitis
Meningoencephalitis
Neuroleptic malignant syndrome
Meninges