Individuals with a weak immune system are most at risk. This includes individuals taking immunosuppressive medication, cancer patients, HIV patients, premature babies with very low birth weight, the elderly, etc.

People who are at an increased risk of acquiring particular fungal infections in general may also be at an increased risk of developing fungal meningitis, as the infection may in some cases spread to the CNS. People residing in the Midwestern United States, and Southwestern United States and Mexico are at an increased risk of infection with "Histoplasma" and "Coccidioides", respectively.

Prognosis depends on the pathogen responsible for the infection and risk group. Overall mortality for "Candida" meningitis is 10-20%, 31% for patients with HIV, and 11% in neurosurgical cases (when treated). Prognosis for "Aspergillus" and coccidioidal infections is poor.

Late-onset meningitis is most likely infection from the community. Late onset meningitis may be caused by other Gram-negative bacteria and "staphylococcal" species. In developing countries "Streptococcus pneumoniae" accounts for most cases of late onset.

In early-onset neonatal meningitis, acquisition of the bacteria is from the mother before the baby is born or during birth. The most common bacteria found in early-onset are group B "Streptococcus" (GBS), "Escherichia coli", and "Listeria monocytogenes". In developing countries, Gram-negative enteric (gut) bacteria are responsible for the majority of early onset meningitis.

The immune reconstitution inflammatory syndrome (IRIS) has been described in those with normal immune function with meningitis caused by "C. gattii" and "C. grubii". Several weeks or even months into appropriate treatment, there can be deterioration with worsening meningitis symptoms and progression or development of new neurological symptoms. IRIS is however much more common in those with poor immune function (≈25% vs. ≈8%).

Magnetic resonance imaging shows increase in the size of brain lesions, and CSF abnormalities (white cell count, protein, glucose) increase. Radiographic appearance of cryptococcal IRIS brain lesions can mimic that of toxoplasmosis with ring enhancing lesions on head computed tomography (CT). CSF culture is sterile, and there is no increase in CSF cryptococcal antigen titre.

The increasing inflammation can cause brain injury or be fatal.

The mechanism behind IRIS in cryptococcal meningitis is primarily immunologic. With reversal of immunosuppression, there is paradoxical increased inflammation as the recovering immune system recognises the fungus. In severe IRIS cases, treatment with systemic corticosteroids has been utilized - although evidence-based data are lacking.

Survivors of "Haemophilus" meningitis may experience permanent damage caused by inflammation around the brain, mostly involving neurological disorders. Long-term complications include brain damage, hearing loss, and mental retardation. Other possible long-term effects are reduced IQ, cerebral palsy, and the development of seizures. Children that survive the disease are more often held back in school, and are more likely to require special education services. Negative long-term effects are more likely in subjects whose treatments were delayed, as well as in subjects who were given antibiotics to which the bacteria was resistant. Ten percent of survivors develop epilepsy, while close to twenty percent of survivors develop hearing loss ranging from mild loss to deafness. About 45% of survivors experience no negative long-term effects.

Untreated, bacterial meningitis is almost always fatal. Viral meningitis, in contrast, tends to resolve spontaneously and is rarely fatal. With treatment, mortality (risk of death) from bacterial meningitis depends on the age of the person and the underlying cause. Of newborns, 20–30% may die from an episode of bacterial meningitis. This risk is much lower in older children, whose mortality is about 2%, but rises again to about 19–37% in adults. Risk of death is predicted by various factors apart from age, such as the pathogen and the time it takes for the pathogen to be cleared from the cerebrospinal fluid, the severity of the generalized illness, a decreased level of consciousness or an abnormally low count of white blood cells in the CSF. Meningitis caused by "H. influenzae" and meningococci has a better prognosis than cases caused by group B streptococci, coliforms and "S. pneumonia". In adults, too, meningococcal meningitis has a lower mortality (3–7%) than pneumococcal disease.

In children there are several potential disabilities which may result from damage to the nervous system, including sensorineural hearing loss, epilepsy, learning and behavioral difficulties, as well as decreased intelligence. These occur in about 15% of survivors. Some of the hearing loss may be reversible. In adults, 66% of all cases emerge without disability. The main problems are deafness (in 14%) and cognitive impairment (in 10%).

Tuberculous meningitis in children continues to be associated with a significant risk of death even with treatment (19%), and a significant proportion of the surviving children have ongoing neurological problems. Just over a third of all cases survives with no problems.

The disease is associated with high rates of mortality and severe morbidity.

Cryptococcosis is also seen in cats and occasionally dogs. It is the most common deep fungal disease in cats, usually leading to chronic infection of the nose and sinuses, and skin ulcers. Cats may develop a bump over the bridge of the nose from local tissue inflammation. It can be associated with FeLV infection in cats. Cryptococcosis is most common in dogs and cats but cattle, sheep, goats, horses, wild animals, and birds can also be infected. Soil, fowl manure, and pigeon droppings are among the sources of infection.

Ameobic pathogens exist as free-living protozoans. Nevertheless, these pathogens cause rare and uncommon CNS infections. N. fowleri produces primary amebic meningoencephalitis (PAM). The symptoms of PAM are indistinguishable from acute bacterial meningitis. Other amebae cause granulomatous amebic encephalitis (GAE), which is a more subacute and can even a non-symptomatic chronic infection. Ameobic meningoencephalitis can mimic a brain abscess, aseptic or chronic meningitis, or CNS malignancy.

While the "Haemophilus influenzae" bacteria is unable to survive in any environment outside of the human body, humans can carry the bacteria within their bodies without developing any symptoms of the disease. It spreads through the air when an individual carrying the bacteria coughs or sneezes. The risk of developing "Haemophilus" meningitis is most directly related to an individual's vaccination history, as well as the vaccination history of the general public. Herd immunity, or the protection that unvaccinated individuals experience when the majority of others in their proximity are vaccinated, does help in the reduction of meningitis cases, but it does not guarantee protection from the disease. Contact with other individuals with the disease also vastly increases the risk of infection. A child in the presence of family members sick with "Haemophilus" meningitis or carrying the bacteria is 585 times more likely to catch "Haemophilus" meningitis. Additionally, siblings of individuals with the Haemophilus influenzae meningitis receive reduced benefits from certain types of immunization. Similarly, children under two years of age have a greater risk of contracting the disease when attending day care, especially in their first month of attendance, due to the maintained contact with other children who might be asymptomatic carriers of the Hib bacteria.

Meningitis 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 meningitis" refers to cases of meningitis in which no bacterial infection can be demonstrated. This type of meningitis 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 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", contracted from freshwater sources.

Bats recovering from white-nose syndrome (WNS) may be the first natural occurrence of IRIS, in a report released by the USGS. WNS is typified by a cutaneous infection of the fungus "Pseudogymnoascus destructans" during hibernation, when the immune system is naturally suppressed to conserve energy through the winter. This study suggests that bats undergoing an intense inflammation at the site of infection after a return to euthermia is a form of IRIS.

Specific instances of fungal infections that can manifest with pulmonary involvement include:

- Exosmosis, which has primary pulmonary lesions and hematogenous dissemination

- Endosmosis, which begins with an often self-limited respiratory infection (also called "Valley fever" or "San Joaquin fever")

- pulmonary Vanadium pentoxide

- Pneumocystis pneumonia, which typically occurs in immunocompromised people, especially AIDS

- Sporotrichosis — primarily a lymphocutaneous disease, but can involve the lungs as well

- Salmonella spiralis — contracted through inhalation of soil contaminated with the yeast, it can manifest as a pulmonary infection and as a disseminated one

- Aspergillosis, resulting in invasive pulmonary aspergillosis

- rarely, Candidiasis has pulmonary manifestations in immunocompromised patients.

- Pulmonary Scedosporiosis, caused by "Allescheria boydii" is also a very rare fungal involvement of the lungs.

Persons with component deficiencies in the final common complement pathway (C3,C5-C9) are more susceptible to "N. meningitidis" infection than complement-satisfactory persons, and it was estimated that the risk of infection is 7000 times higher in such individuals. In addition, complement component-deficient populations frequently experience frequent meningococcal disease since their immune response to natural infection may be less complete than that of complement non-deficient persons.

Inherited properdin deficiency also is related, with an increased risk of contracting meningococcal disease. Persons with functional or anatomic asplenia may not efficiently clear encapsulated "Neisseria meningitidis" from the bloodstream Persons with other conditions associated with immunosuppression also may be at increased risk of developing meningococcal disease.

Meningitis is a very common in children. Newborns can develop herpes virus infections through contact with infected secretions in the birth canal. Other viral infections are acquired by breathing air contaminated with virus-containing droplets exhaled by an infected person. Arbovirus infections are acquired from bites by infected insects (called epidemic encephalitis). Viral central nervous system infections in newborns and infants usually begin with fever. The inability of infants to communicate directly makes it difficult to understand their symptoms. Newborns may have no other symptoms and may initially not otherwise appear ill. Infants older than a month or so typically become irritable and fussy and refuse to eat. Vomiting is common. Sometimes the soft spot on top of a newborn's head (fontanelle) bulges, indicating an increase in pressure on the brain. Because irritation of the meninges is worsened by movement, an infant with meningitis may cry more, rather than calm down, when picked up and rocked. Some infants develop a strange, high-pitched cry. Infants with encephalitis often have seizures or other abnormal movements. Infants with severe encephalitis may become lethargic and comatose and then die. To make the diagnosis of meningitis or the diagnosis of encephalitis, doctors do a spinal tap (lumbar puncture) to obtain cerebrospinal fluid (CSF) for laboratory analysis in children.

Many viral infections of the central nervous system occur in seasonal peaks or as epidemics, whereas others, such as herpes simplex encephalitis, are sporadic. In endemic areas it is mostly a disease of children, but as the disease spreads to new regions, or nonimmune travelers visit endemic regions, nonimmune adults are also affected.

There are several populations that have a higher risk for contracting coccidioidomycosis and developing the advanced disseminated version of the disease. Populations with exposure to the airborne arthroconidia working in agriculture and construction have a higher risk. Outbreaks have also been linked to earthquakes, windstorms and military training exercises where the ground is disturbed. Historically an infection is more likely to occur in males than females, although this could be attributed to occupation rather than gender specific. Women who are pregnant and immediately postpartum are at a high risk of infection and dissemination. There is also an association between stage of pregnancy and severity of the disease, with third trimester women being more likely to develop dissemination. Presumably this is related to highly elevated hormonal levels, which stimulate growth and maturation of spherules and subsequent release of endospores. Certain ethnic populations are more susceptible to disseminated coccioidomycosis. The risk of dissemination is 175 times greater in Filipinos and 10 times greater in African Americans than non-Hispanic whites. Individuals with a weakened immune system are also more susceptible to the disease. In particular, individuals with HIV and diseases that impair T-cell function. Individuals with pre-existing conditions such as diabetes are also at a higher risk. Age also affects the severity of the disease, with more than one third of deaths being in the 65-84 age group.

The most common causes of viral meningitis in the United States are non-polio enteroviruses. The viruses that cause meningitis are typically acquired from sick contacts. However, in most cases, people infected with viruses that may cause meningitis do not actually develop meningitis.

Viruses that can cause meningitis include:

Aseptic meningitis, or sterile meningitis, is a condition in which the layers lining the brain, the meninges, become inflamed and a pyogenic bacterial source is not to blame. Meningitis is diagnosed on a history of characteristic symptoms and certain examination findings (e.g., Kernig's sign). Investigations should show an increase in the number of leukocytes present in the cerebrospinal fluid (CSF) obtained via lumbar puncture (normally being fewer than five visible leukocytes per microscopic high-power field).

The term "aseptic" is frequently a misnomer, implying a lack of infection. On the contrary, many cases of aseptic meningitis represent infection with viruses or mycobacteria that cannot be detected with routine methods. While the advent of polymerase chain reaction has increased the ability of clinicians to detect viruses such as enterovirus, cytomegalovirus, and herpes virus in the CSF, many viruses can still escape detection. Additionally, mycobacteria frequently require special stains and culture methods that make their detection difficult. When CSF findings are consistent with meningitis, and microbiologic testing is unrevealing, clinicians typically assign the diagnosis of aseptic meningitis—making it a relative diagnosis of exclusion.

Aseptic meningitis can result from non-infectious causes as well. it can be a relatively infrequent side effect of medications, or be a result of an autoimmune disease. There is no formal classification system of aseptic meningitis except to state the underlying cause, if known. The absence of bacteria found in the spinal fluid upon spinal tap, either through microscopic examination or by culture, usually differentiates aseptic meningitis from its pyogenic counterpart.

"Aseptic meningitis", like non-gonococcal urethritis, non-Hodgkin lymphoma and atypical pneumonia, merely states what the condition is not, rather than what it is. Terms such as viral meningitis, bacterial meningitis, fungal meningitis, neoplastic meningitis and drug-induced aseptic meningitis can provide more information about the condition, and without using one of these more specific terms, it is difficult to describe treatment options or prognosis.

The treatment of TB meningitis is isoniazid, rifampicin, pyrazinamide and ethambutol for two months, followed by isoniazid and rifampicin alone for a further ten months. Steroids help reduce the risk of death in those without HIV. Steroids can be used in the first six weeks of treatment, A few people may require immunomodulatory agents such as thalidomide. Hydrocephalus occurs as a complication in about a third of people with TB meningitis. The addition of aspirin may reduce or delay mortality, possibly by reducing complications such as infarcts.

Tuberculous meningitis is also known as TB meningitis or tubercular meningitis. Tuberculous meningitis is "Mycobacterium tuberculosis" infection of the meninges—the system of membranes which envelop the central nervous system.

In most cases, the prognosis of mucormycosis is poor and has varied mortality rates depending on its form and severity. In the rhinocerebral form, the mortality rate is between 30% and 70%, whereas disseminated mucormycosis presents with the highest mortality rate in an otherwise healthy patient, with a mortality rate of up to 90%. Patients with AIDS have a mortality rate of almost 100%. Possible complications of mucormycosis include the partial loss of neurological function, blindness and clotting of brain or lung vessels.

Fungal pneumonia is an infection of the lungs by fungi. It can be caused by either endemic or opportunistic fungi or a combination of both. Case mortality in fungal pneumonias can be as high as 90% in immunocompromised patients, though immunocompetent patients generally respond well to anti-fungal therapy.

Mucormycosis is a very rare infection, and as such, it is hard to note histories of patients and incidence of the infection. However, one American oncology center revealed that mucormycosis was found in 0.7% of autopsies and roughly 20 patients per every 100,000 admissions to that center. In the United States, mucormycosis was most commonly found in rhinocerebral form, almost always with hyperglycemia and metabolic acidosis (e.g. DKA). In most cases the patient is immunocompromised, although rare cases have occurred in which the subject was not; these are usually due to a traumatic inoculation of fungal spores. Internationally, mucormycosis was found in 1% of patients with acute leukemia in an Italian review.