Histoplasmosis is not contagious. It does not spread from person to person, from person to pet, or from animals to people. You cannot catch it by being near someone who is infected, sharing a room with them, or touching them. Every case of histoplasmosis comes from the same place: inhaling fungal spores from the environment.
How People Actually Get Histoplasmosis
The fungus that causes histoplasmosis lives in soil, especially soil enriched with bird or bat droppings. The droppings don’t contain the fungus themselves but act as fertilizer that helps it thrive. When contaminated soil or material is disturbed, tiny spores become airborne. These spores are small enough to bypass the nose, throat, and upper airways entirely, traveling deep into the lungs where they settle into the air sacs.
Once inside the lungs, the spores transform from a mold form into a yeast form. This shape-shift is what allows the fungus to survive inside the body and interact with immune cells. The infection starts and stays in the lungs for most people, though in some cases it can spread to other organs.
Unlike bats, birds don’t actually get infected with the fungus. Bats can become infected and may excrete the organism in their droppings, which is why bat-heavy environments carry extra risk.
Activities That Put You at Risk
Any activity that kicks up contaminated dust or soil can expose you. Construction and demolition work near old buildings where birds have roosted is a well-known source. Cleaning chicken coops, disturbing attics with bat colonies, and landscaping in endemic areas all carry risk. Caving is another significant exposure route. A 2025 outbreak among travelers returning from a cave tour in Costa Rica sickened a group who had crawled through tight spaces with direct contact with bat droppings. That outbreak prompted the U.S. Embassy in Costa Rica to issue a public health alert about caving risks.
The common thread is always the same: disturbed soil or accumulated droppings in an enclosed or semi-enclosed space, not contact with another person.
Where It’s Most Common
In the United States, histoplasmosis is concentrated around the Ohio and Mississippi River valleys. An estimated 60 to 90 percent of people living in those regions are exposed to the fungus at some point in their lives. Most never realize it. The fungus also exists in parts of Central America, South America, Africa, and Asia.
What It Feels Like and How Long It Takes
Most people who inhale the spores never develop symptoms at all. When symptoms do appear, they typically resemble a mild flu: fever, cough, fatigue, body aches, and chest discomfort. In a large outbreak study, more than 75 percent of people who got sick were ill for a week or less, and all recovered completely within two months without any treatment.
Moderate cases can drag on longer with worsening cough, persistent fever, and expanding areas of inflammation visible on chest imaging. These cases sometimes warrant antifungal treatment, depending on how severe the symptoms become and how long they last.
Who Faces Serious Complications
For most healthy people, histoplasmosis resolves on its own. The real danger is disseminated histoplasmosis, where the infection spreads beyond the lungs to organs like the liver, spleen, bone marrow, or brain. This form occurs primarily in people with weakened immune systems: those living with HIV/AIDS, people on chemotherapy, organ transplant recipients taking immunosuppressive drugs, and patients on long courses of steroids. There is also growing recognition that prolonged steroid treatment during severe respiratory infections like COVID-19 pneumonia may increase susceptibility to disseminated fungal disease.
People with these risk factors who live in or travel to areas where the fungus is common should be aware that even a mild exposure could lead to a more serious infection.
When Treatment Is Needed
Updated 2025 guidelines from the Infectious Diseases Society of America confirm that most mild to moderate cases in otherwise healthy people do not need antifungal medication. Treatment is typically reserved for cases where symptoms are prolonged, worsening, or the person has a condition that weakens their immune defenses. Asymptomatic lung nodules found incidentally on imaging generally don’t require treatment either, though people at higher risk for dissemination may be monitored more closely or treated as a precaution.
How It’s Diagnosed
Testing for histoplasmosis has become quite accurate. Antigen testing of urine or blood is the most sensitive method, detecting the infection in about 98 percent of confirmed cases. Antibody testing catches around 80 percent of cases and works best in people with healthy immune systems. Combining both antigen and antibody tests pushes sensitivity to over 99 percent. Fungal culture is still used but is less reliable on its own and takes longer to produce results.
Protecting Yourself in High-Risk Settings
If you work in construction, pest control, demolition, or any job that involves disturbing soil or old structures where birds or bats have lived, respiratory protection matters. The CDC recommends particulate-filtering respirators for anyone working in areas with known or suspected contamination. For high-risk activities like cleaning up a site already linked to illness, a powered air-purifying respirator or full-facepiece respirator with high-efficiency filters is preferred. These provide two to five times the protection of a standard N95 mask. If those aren’t available, an N95 still offers meaningful protection and is far better than nothing.
For recreational cavers, the practical advice is simpler: avoid caves with visible bat colonies or heavy droppings, and if you do enter one, wear at minimum an N95 respirator. Wetting down dusty surfaces before disturbing them can also reduce the number of spores that become airborne, though this isn’t always feasible underground.