An AIDS patient with a CD4 count in the range 100 to 50 cells/mm3 complains of headache and neck stiffness and appears disoriented. The possibility of fungal meningitis is considered and tests for the common fungal etiology of meningitis ordered. Tests included direct examination of spinal fluid for the organism and serology. The purpose of the serology test is detection of

Cryptococcus neoformans is the most common cause of meningitis in AIDS patients. The frequency has declined in areas with robust anti-retroviral treatment but can still occur in patients receiving treatment. Patients immunocompromised by other factors, for example maintained on corticosteroids, are also at risk. The organism is inhaled as a desiccated, minimally encapsulated organism or perhaps a basidiospore often after disturbance of soil rich in avian feces such as pigeon guano. The capsule can enlarge upon reaching the lung. Initial pulmonary infection may be asymptomatic and exposure may occur early in life. Symptomatic infection may mimic an influenza-like respiratory infection and resolve spontaneously. Particularly in immunocompromised patients the organism may multiply and disseminate with a predilection for the CNS causing meningoencephalitis. This is often the presenting complaint. The organism grows as encapsulated yeast that, in culture and cerebrospinal fluid, may be highlighted by mounting in India ink. Frequently, detection of capsular polysaccharide antigen is used as a diagnostic tool. There are commercial kits based on latex agglutination or enzyme immunoassay that can be used to detect antigen in serum or cerebrospinal fluid.