Cryptococcosis is a fungal infection caused by inhaling microscopic yeast cells found in the environment, particularly in soil contaminated with bird droppings. It primarily affects the lungs and brain, and it is one of the most dangerous opportunistic infections for people with weakened immune systems. Each year, an estimated 152,000 cases of cryptococcal meningitis occur among people living with HIV worldwide, leading to roughly 112,000 deaths, most of them in sub-Saharan Africa.
What Causes Cryptococcosis
Two species of the fungus Cryptococcus are responsible: C. neoformans and C. gattii. Both belong to a group of fungi that reproduce as yeast cells, but they stand apart from other disease-causing yeasts because they produce a protective outer capsule made of sugar molecules, generate a dark pigment called melanin, and break down urea. All three traits help the fungus survive inside the human body.
C. neoformans is found worldwide, thriving in soil enriched by pigeon and other bird droppings. Birds, especially pigeons, carry the fungus in their gut and spread it across wide geographic areas through their excrement. The soil beneath roosting sites tends to be acidic and nutrient-rich, creating an ideal habitat. C. gattii has historically been linked to tropical and subtropical regions and certain tree species, though outbreaks in temperate areas like the Pacific Northwest of North America have expanded its known range.
Importantly, cryptococcosis does not spread from person to person. You get it by breathing in dried yeast cells or spores that have become airborne from contaminated soil or decaying organic matter. Once inhaled, these tiny particles are small enough to travel deep into the lungs and settle in the air sacs.
Who Is Most at Risk
The vast majority of serious cryptococcal infections occur in people whose immune systems are severely compromised. HIV is the single biggest risk factor. About 90% of cryptococcal cases in people with HIV occur when their CD4 cell count, a key marker of immune function, drops below 100 cells per cubic millimeter (a healthy count is typically 500 to 1,500). At CD4 counts below 50, nearly 1 in 25 people with HIV already carries detectable fungal proteins in their blood, signaling early infection even before symptoms appear.
Other groups at elevated risk include organ transplant recipients taking immunosuppressive medications, people on long-term corticosteroids, and those with certain cancers or autoimmune conditions that weaken immune defenses. In parts of the world without reliable access to antiretroviral therapy (ART), cryptococcal meningitis remains a leading cause of death among people with HIV.
C. gattii behaves differently from C. neoformans in one striking way: it can cause serious disease in people with fully healthy immune systems. In studies comparing the two species, all C. gattii infections occurred in otherwise healthy individuals, while 90% of C. neoformans infections occurred in immunosuppressed patients. People infected with C. gattii were less likely to die but more likely to develop lasting neurological problems that sometimes required surgery and prolonged treatment.
How Cryptococcosis Affects the Body
Lung Infection
Because the fungus enters through the airways, the lungs are usually the first organ affected. Cough is the most common symptom, present in over 80% of patients with pulmonary cryptococcosis. Chest imaging typically shows nodules in the lungs. In immunocompromised patients, those nodules are more likely to develop cavities or progress to larger areas of dense inflammation, while immunocompetent patients tend to have more contained, nodular disease. Some people with healthy immune systems clear the infection on their own without ever knowing they had it.
Brain and Spinal Cord Infection
The most feared complication is cryptococcal meningitis, an infection of the membranes surrounding the brain and spinal cord. This happens when the fungus spreads from the lungs through the bloodstream. Sometimes meningitis is the first sign of illness, with no obvious lung symptoms beforehand. Symptoms include headache, fever, nausea and vomiting, neck pain, confusion or behavioral changes, and sensitivity to light. Without treatment, cryptococcal meningitis is fatal.
In immunosuppressed patients, the fungus can also be found in the blood and urine, reflecting widespread dissemination throughout the body.
How Cryptococcosis Is Diagnosed
The gold standard for confirming cryptococcal meningitis is examining spinal fluid. Doctors perform a lumbar puncture (spinal tap) and test the fluid in two classic ways: staining it with India ink, which outlines the fungus’s distinctive capsule under a microscope, and growing the organism in culture. Culture is reliable but requires several days of incubation. India ink staining is faster but can miss cases when the fungal count is low, with sensitivity dropping as low as 86% in some studies.
A newer approach, the cryptococcal antigen lateral flow assay (CrAg LFA), has largely transformed diagnosis. Approved by the FDA in 2011, this rapid point-of-care test detects fungal proteins in blood or spinal fluid with about 99.5% sensitivity and 99.5% specificity when used on spinal fluid. It delivers results in minutes, requires minimal lab equipment, and is especially valuable in resource-limited settings where culture facilities may not be available. For people newly diagnosed with HIV whose CD4 count is 200 or below, routine blood screening with the CrAg test is recommended to catch early infection before meningitis develops.
Treatment
Treating cryptococcal meningitis is a multi-phase process. Current guidelines from the WHO (2022) and joint U.S. guidelines (2024) recommend starting with an aggressive induction phase: a single high dose of a powerful antifungal given intravenously, combined with two weeks of two oral antifungal medications. This initial assault aims to rapidly reduce the number of fungal organisms in the brain and spinal fluid.
After induction, patients move to a consolidation phase of eight weeks on a lower dose of oral antifungal medication, followed by a maintenance phase that continues until the immune system has recovered sufficiently. For people with HIV, maintenance therapy can typically be stopped once their CD4 count rises above 100 and their viral load is well controlled on ART. If CD4 counts drop below 100 again, maintenance therapy restarts.
Treatment for pulmonary cryptococcosis without brain involvement is generally less intensive and may involve oral antifungal medication alone, depending on immune status and the severity of lung disease.
A Dangerous Complication During Recovery
For people with HIV who begin antiretroviral therapy after a cryptococcal infection, there is a paradoxical risk. As the immune system rebuilds, it can mount an excessive inflammatory response against fungal material still present in the body. This is called immune reconstitution inflammatory syndrome, or IRIS, and it develops in up to 30% of patients with cryptococcal meningitis.
IRIS typically occurs within the first eight weeks of starting ART, though delayed cases have been reported years later. The mechanism works roughly like this: before ART, the weakened immune system cannot clear the fungus effectively. Once ART restores immune cell numbers and function, the newly active immune cells encounter leftover fungal proteins and react aggressively. The resulting inflammation, particularly in the brain and spinal fluid, can cause symptoms that mimic a worsening infection even though fungal cultures are negative. Patients may experience renewed headache, fever, and neurological decline. Managing this complication requires careful timing of when to start ART relative to antifungal treatment, balancing the urgency of immune recovery against the risk of triggering a damaging inflammatory response.
Global Impact
Cryptococcosis remains a disease shaped largely by access to healthcare. In high-income countries where ART and antifungal medications are widely available, it has become relatively uncommon. In sub-Saharan Africa and other regions with limited access to HIV treatment and diagnostic tools, it remains one of the top killers of people living with HIV. The 112,000 annual deaths from cryptococcal meningitis place it alongside tuberculosis as a leading infectious cause of death in this population. For C. gattii infections, which affect healthy individuals in specific geographic regions, reported death rates range from 13% to 33% depending on the study and location.