A rare fungal infection that can invade the human brain appears to be detected more often in the United States, prompting scientists to investigate what may be driving the increase.
The fungus, Cladophialophora bantiana, is unusual because it has a strong tendency to attack the central nervous system. It can cause brain abscesses and has been associated with a case-fatality rate of 60% to 70%, according to a new report from the US Centers for Disease Control and Prevention (CDC).
Yet one of the most troubling aspects of the fungus is what researchers still do not know. Researchers have not clearly established its natural environmental reservoir, and the precise route by which it reaches the brain remains uncertain.
The CDC report, published October 2 in the Morbidity and Mortality Weekly Report, found that detections of C. bantiana increased sharply between 2023 and 2025. Researchers say the reasons for the apparent rise remain unknown.
How many cases of C. bantiana have been detected?
Researchers examined records from a specialized clinical reference laboratory at the University of Washington that performs molecular testing for fungal infections.
Between January 2009 and July 2026, the laboratory identified 48 confirmed cases of C. bantiana infection and one probable case. The cases originated in 21 US states and Washington, DC.
Most of the detections involved the central nervous system. Forty-three of the 49 cases, or 88%, were identified in intracranial tissue, while six cases involved lung specimens.
The annual numbers were relatively low for many years. Between 2009 and 2023, the laboratory recorded between one and five cases annually.
That changed in 2024, when eight cases were detected, followed by 14 cases in 2025.
Researchers found the increase was not explained simply by a larger number of specimens being sent to the laboratory. When they normalized C. bantiana detections against all central nervous system specimens submitted for fungal PCR testing, the frequency also rose substantially.
Between 2017 and 2023, the laboratory detected an average of 8.4 C. bantiana cases per 1,000 CNS specimens. By 2025, the figure had risen to 26 per 1,000.
Why is this fungus so dangerous?
C. bantiana belongs to a group of darkly pigmented molds known as dematiaceous fungi. It is described as neurotropic, meaning it has a particular tendency to affect the nervous system.
The infection most commonly manifests as a brain abscess, a pocket of infected material that can place pressure on surrounding brain tissue.
Unlike many invasive fungal infections, C. bantiana does not exclusively threaten people with weakened immune systems.
The CDC says approximately half of people who develop C. bantiana infections have no preceding immunocompromising condition. Historical medical literature has likewise documented serious brain infections in people who were otherwise considered immunocompetent.
That characteristic makes the pathogen especially difficult to understand because conventional assumptions about who is most vulnerable do not always apply.
How does C. bantiana get into the brain?
This is one of the biggest unanswered questions surrounding the fungus.
Scientists have proposed several possible routes of infection. One leading theory is that people inhale fungal spores, after which the organism enters the bloodstream and eventually reaches the brain.
Another possibility is direct inoculation through the skin after an injury. Direct spread from nearby structures such as the sinuses has also been proposed.
None of these routes has been definitively established as the usual way humans acquire the infection.
Adding to the mystery, most patients with brain infections have no obvious infection elsewhere in the body and no clear history of exposure that would explain how the organism entered their system.
The CDC also notes that C. bantiana has only rarely been isolated from environmental sources, making it difficult for researchers to identify where people are encountering it.
Can healthy people get this fungal brain infection?
Yes.
Although immunocompromised people can develop invasive fungal infections, C. bantiana has a notable history of causing disease in people without an identified immune deficiency.
That does not mean healthy people are at high risk of developing the disease. The infection remains extremely rare.
Instead, the significance is that an apparently healthy immune system does not guarantee protection against this particular organism.
Researchers believe the fungus’s ability to reach and persist in the brain may be linked to biological characteristics that distinguish it from many other environmental molds.
What symptoms can the infection cause?
Once C. bantiana reaches the brain, the resulting infection can resemble other types of brain abscess or intracranial disease.
Reported symptoms include persistent headaches, seizures, weakness affecting one side or particular parts of the body, changes in mental status, speech problems, fever, vomiting and other neurological abnormalities.
Because these symptoms can occur in numerous neurological conditions, diagnosis can be difficult.
Brain imaging may reveal an abscess or lesion, but imaging alone cannot reliably establish that C. bantiana is responsible. Laboratory testing of tissue or other clinical specimens may be required to identify the fungus.
Earlier reports from India and other countries have described cases in which fungal brain abscesses were initially mistaken for conditions such as tuberculosis or tumors, illustrating the diagnostic challenges involved.
Is the fungus spreading from person to person?
There is no evidence from the CDC report that C. bantiana is spreading through ordinary person-to-person transmission.
The suspected mechanisms involve environmental exposure followed by infection and, in some cases, spread through the bloodstream. Because the natural reservoir and precise infection route remain uncertain, researchers are still trying to determine where the organism comes from and how people encounter it.
The current investigation therefore should not be interpreted as evidence of a contagious outbreak.
Is the number of infections really increasing?
Researchers are careful about this point.
The CDC report does not provide a nationwide incidence estimate because the United States does not have routine national surveillance for C. bantiana infections.
Instead, the study examined detections at one specialized reference laboratory. The researchers adjusted the numbers for the volume of CNS specimens submitted for testing and still found a substantial increase in the frequency of C. bantiana detections.
That makes the signal concerning, but it does not establish exactly how many people across the United States are infected each year.
The report’s authors say systematic reporting is needed to determine whether the increase seen at the University of Washington reflects a broader national trend.
What could be behind the increase?
At this stage, researchers do not have a confirmed explanation.
The CDC says the reasons for the apparent increase are unknown. Scientists are considering the possibility that changes in environmental exposure, diagnostic practices, laboratory detection, population factors or the organism itself could contribute.
Another possibility is that infections have historically been missed or misclassified because they are so rare and difficult to diagnose.
Researchers have also called for additional investigation into epidemiological risk factors and whether changes in the fungus’s virulence could play a role.
For now, however, those possibilities remain hypotheses rather than established causes.
A fungus scientists have struggled to understand for decades
C. bantiana has been known to medicine for more than a century, yet many basic questions remain unanswered.
The first reported brain abscess associated with the fungus dates to 1911. A century later, scientists still do not have a clear picture of its natural habitat or the usual pathway by which it reaches the brain.
That gap in knowledge becomes more important when detections rise.
Without a reliable understanding of where the fungus lives, how humans are exposed or which groups are most vulnerable, public-health officials have limited ability to estimate risk or design targeted prevention measures.
Why the new CDC report matters
The most important message from the study is not that a widespread fungal outbreak is underway. It is that a rare and potentially devastating infection is being detected more often at a specialized US laboratory, and researchers do not yet know why.
That uncertainty is precisely why the CDC and University of Washington researchers are calling for better surveillance.
Doctors may also need to keep C. bantiana in mind when evaluating patients with brain abscesses, including patients without obvious immune problems.
The infection’s rarity means it remains an unlikely diagnosis for most people. But when it does occur, the consequences can be severe, and early recognition may be critical.
The CDC report says patients with confirmed infection require prompt antifungal therapy and notes that surgical excision or drainage of brain abscesses may improve outcomes in appropriate cases.
For scientists, the next challenge is figuring out whether the recent rise represents a temporary fluctuation, improved recognition or a genuine change in the epidemiology of a fungus that has remained mysterious for generations.
