On the evening of October 14, a post on X told almost half a million people that a "deadly brain-infecting fungus is on the rise in the US, with fatality rates reaching 70%," and that "scientists are unsure of how it spreads and why cases are steadily rising."
The post names no fungus and no study. I traced it, and it is real. It is also smaller, stranger and less contagious than it sounds.
1. What it is
The fungus is Cladophialophora bantiana, a dark mold that has a habit of growing in the human brain. The source is a short CDC report, "Notes from the Field: Molecular Detections of Cladophialophora bantiana at a Clinical Reference Laboratory, United States, 2009 to 2026," published in CDC's Morbidity and Mortality Weekly Report on October 2, 2026 (MMWR volume 75, doi:10.15585/mmwr.mm7540e1). The authors are four researchers at the University of Washington in Seattle and one from CDC's Mycotic Diseases Branch.
2. What the report found
From January 2009 to July 1, 2026, the University of Washington's lab found 48 confirmed cases and one probable case of the fungus in samples sent in for broad fungal DNA testing. Of those 49, 43 (88%) were in brain tissue and six (12%) were in lung samples. The median patient age was 68.
The lab found one to five cases a year from 2009 through 2023. It found eight in 2024 and 14 in 2025.
The lab also checked whether it was simply testing more. From 2017 to 2023 it averaged 8.4 cases per 1,000 brain and spinal specimens tested. In 2025 that reached 26 per 1,000. The authors say the rise is "not attributable to an increase in laboratory referral volume alone," and describe detections as up "approximately fourfold" from 2023 to 2025.
The cases came from 21 states and Washington, DC. The most came from California (six), Florida (five), Texas (five) and Pennsylvania (four).
The authors call it an "apparent increase" and say the reasons "are unknown."
3. Checking the claim, piece by piece
"Deadly brain-infecting fungus." Confirmed. CDC's report calls it "highly virulent" and says it "predominantly causes brain abscesses," pockets of infection inside the brain.
"On the rise in the US." True, with an asterisk. The rise is real at one lab, and that lab takes samples from across the country. But there is no national count. The report says plainly: "No routine surveillance is available." Nobody knows how many cases the country has in total.
"Fatality rates reaching 70%." True, with a bigger asterisk. The 60% to 70% figure in the CDC report comes from a 2017 review of published cases worldwide (Kantarcioglu and colleagues, Medical Mycology, doi:10.1093/mmy/myw124), which found 65% of brain cases died. CDC adds that the rate holds "even with aggressive therapy." It is a figure for people who already have the brain infection. It is not the share of the US cases in this report who died: the lab had outcome data for only one of the 49 patients, who died. A 2016 review of 124 cases (Chakrabarti and colleagues, Medical Mycology), as summarized by the Public Health Agency of Canada, found deaths ran higher in people with weakened immune systems (77%) than in people with healthy ones (56%).
"Scientists are unsure of how it spreads." Partly true, and easy to misread. CDC says the route of infection is not "fully understood," but the presumed route is breathing in spores or getting them into a skin wound, after which the fungus travels to the brain. That is how a mold from the environment gets into one person. It is not person-to-person spread. None of the sources I checked, CDC included, documents the fungus passing from one person to another.
"Why cases are steadily rising." The why is unknown, as the post says. "Steadily" is off: case counts were flat for 15 years, then jumped in 2024 and 2025.
4. What you should actually know
This is rare. The Public Health Agency of Canada counts fewer than 200 brain cases ever reported worldwide, across 26 countries. The CDC report covers 49 in 17 years at one major US lab.
It does not only strike the sick. CDC says about half of patients have no prior immune problem. CDC's general guidance is that people with weakened immune systems are at higher risk for most fungal infections. Men are affected more often than women, according to Canada's health agency.
Brain abscesses caused by this fungus tend to show up as headache, weakness on one side of the body, seizures, nausea and vomiting, and fever, according to the Public Health Agency of Canada's summary of the medical literature. Those are symptoms of many illnesses. A severe new headache with weakness, confusion or a seizure needs a doctor fast, whatever the cause.
There is no vaccine. CDC's report says patients need prompt antifungal treatment and that surgery to remove the abscess, when possible, has been linked to better outcomes. It urges doctors to consider the fungus in anyone with signs of a brain abscess.
CDC also notes that fungal infections in general are often misdiagnosed, which can delay the right treatment.
5. What it means
The viral post got the fungus right and the feeling wrong. A mold that kills most of the people it reaches is real, and a fourfold jump at a national reference lab deserves attention. But this is a few dozen cases, not an outbreak, and there is no sign it passes between people.
The unsettling part is the blind spot. Nobody tracks this fungus nationally. Nobody knows where it lives, why it found more brains in 2025, or whether the numbers would look different if more labs looked. The CDC authors are asking for exactly that: systematic reporting, so the next count is a real one.
Sources: CDC MMWR, October 2, 2026 (doi:10.15585/mmwr.mm7540e1); CDC, About Fungal Diseases; Public Health Agency of Canada, Cladophialophora bantiana pathogen safety data sheet; Kantarcioglu et al., Medical Mycology, 2017; Chakrabarti et al., Medical Mycology, 2016. Hero image: Medmyco, Wikimedia Commons, CC BY-SA 4.0. Figures as of October 14, 2026.