Histoplasmosis is far more common than most people realize. The reported incidence sits at 1 to 2 cases per 100,000 people per year in the United States, but that number vastly undercounts the true picture. Only about 1% of people exposed to the fungus ever develop symptoms, and many symptomatic cases are misdiagnosed as bacterial pneumonia. In parts of the Ohio and Mississippi River Valleys, skin testing has shown that 80% to 90% of adults have been exposed to the fungus by age 20.
Reported Cases vs. Actual Exposure
Histoplasmosis is not a nationally reportable disease in the United States. Only 14 states require doctors to report cases, which means the national picture relies on incomplete data. Among those states, the average annual incidence falls between 1 and 2 cases per 100,000 people, with state-level rates ranging from 0 to 4.3 per 100,000.
Those numbers represent only the people who got sick enough to be diagnosed. The CDC estimates that just 1% of people who inhale the fungal spores develop symptoms, typically appearing 3 to 17 days after exposure. The other 99% fight off the infection without ever knowing it happened. Nationwide skin testing conducted in the 1940s and 1950s found that about 20% of young military recruits tested positive for prior exposure. In the most heavily affected regions, that figure climbed to 60% to 90%. Although mass skin testing is no longer performed, these numbers reveal an enormous gap between how many people encounter the fungus and how many ever show up in medical statistics.
Where the Fungus Lives
The fungus that causes histoplasmosis thrives in soil across the central and eastern United States, with the heaviest concentration around the Ohio and Mississippi River Valleys. It grows especially well in soil enriched with bird or bat droppings. Chickens, starlings, grackles, pigeons, and gulls are the bird species most commonly linked to outbreaks. Bats and large roosts or nesting sites can concentrate the fungus in specific areas, though many people who develop histoplasmosis have no known contact with birds or bats at all.
Outside the United States, the fungus is found across Latin America, parts of Africa, and Asia. Brazil and Guatemala have documented particularly high rates among people living with HIV. Some organic fertilizers in Latin America have tested positive for the fungus. Environmental changes may be expanding the regions where the fungus can survive, though firm data on how much the endemic range has shifted is still limited.
How Often It Leads to Hospitalization
For healthy people, histoplasmosis is usually mild or causes no symptoms at all. But it can become serious enough to require hospital care. Between 2001 and 2012, roughly 50,800 histoplasmosis-related hospitalizations occurred in the United States. The annual number nearly doubled over that period, rising from about 2,600 hospitalizations in 2001 to 5,175 in 2012. That translates to a hospitalization rate that climbed from 0.9 to 1.7 per 100,000 people, an average annual increase of nearly 7%.
Among those hospitalized, 4.9% died during their hospital stay. The infection is most dangerous when it spreads beyond the lungs to other organs, a complication called disseminated histoplasmosis that primarily affects people with weakened immune systems.
Who Faces the Highest Risk
People with suppressed immune systems are dramatically more likely to develop severe disease. Among people living with HIV who present with infectious symptoms, the prevalence of confirmed histoplasmosis ranges from 22% in Brazil to 44% in the United States, according to a systematic review published in Medical Mycology. Mortality in this group is staggering, ranging from 21% to 53%. A Guatemalan study found a 7.4% incidence rate of histoplasmosis among people with HIV, higher than previous estimates for that region. Advanced HIV disease is an independent risk factor, meaning the degree of immune suppression directly influences the likelihood of infection and death.
Organ transplant recipients and others on immune-suppressing medications also face elevated risk, though the specific prevalence data for these groups is less well established than for HIV. The fungus can reactivate years after initial exposure if a person’s immune system later becomes compromised, so living in an endemic area at any point in your life creates a lasting vulnerability.
Why So Many Cases Are Missed
Histoplasmosis frequently mimics bacterial or viral pneumonia. Patients typically show up with cough, fever, and fatigue, and clinicians often treat them with antibiotics before considering a fungal cause. The CDC has noted that misdiagnosis leads to inappropriate antibacterial therapy and delayed antifungal treatment. This is a particular problem outside known endemic regions, where doctors may not think to test for it, and it contributes to the significant undercount in official statistics.
The 6% figure is telling in another way: among all fungal meningitis cases in one U.S. study, 6% were caused by the same organism responsible for histoplasmosis. This suggests the fungus is a more significant player in serious infections than its relatively low profile would imply. For comparison, Sweden reports just 0.01 cases per 100,000 people annually, illustrating how dramatically prevalence varies by geography and how much of the global burden concentrates in the Americas.
Putting the Numbers in Perspective
If you live in the Ohio or Mississippi River Valley, the odds that you’ve breathed in histoplasmosis spores at some point are very high. Most people clear the infection without symptoms and develop some degree of immunity. The real danger lies in heavy exposures (disturbing contaminated soil, cleaning old buildings with bird or bat droppings) and in having a compromised immune system. The roughly 5,000 hospitalizations per year in the U.S. represent a small fraction of total exposures, but they reflect a disease that can turn life-threatening when conditions align. The rising hospitalization trend over the past two decades suggests the true burden is growing, whether from expanded geographic range, better detection, or increasing numbers of immunocompromised people in the population.