Fungi cause a wide range of infections in humans, from mild skin irritations to life-threatening bloodstream infections. Globally, fungal infections affect over 1 billion people each year and cause an estimated 3.8 million deaths annually. Here are five of the most significant diseases caused by fungi, how they spread, what they do to the body, and who is most at risk.
1. Candidiasis
Candidiasis is caused by yeasts in the Candida family, most commonly Candida albicans. Most people carry small amounts of Candida on their skin and inside their body without any problems. When the balance tips, usually because of a weakened immune system, antibiotics, or medical devices like catheters, the fungus can overgrow and cause infection.
Mild forms of candidiasis are extremely common. Vaginal yeast infections, oral thrush (white patches in the mouth), and skin fold infections fall into this category. These surface-level infections cause itching, redness, and discomfort but generally respond well to antifungal creams or short courses of oral medication.
The serious version is invasive candidiasis, where Candida enters the bloodstream, joints, or organs. An estimated 25,000 bloodstream Candida infections occur in the United States each year, making it one of the most common hospital-acquired bloodstream infections. About one third of patients with a Candida bloodstream infection die during hospitalization. People over 65 and infants under one year old face the highest risk, particularly if they have surgical wounds, central IV lines, or are on ventilators. Each case adds roughly 3 to 13 extra days in the hospital.
2. Ringworm (Dermatophytosis)
Despite its name, ringworm has nothing to do with worms. It’s a superficial skin infection caused by fungi in three groups: Trichophyton, Epidermophyton, and Microsporum. The hallmark sign is a red, circular, scaly patch on the skin with a clearer center, which is how the infection got its misleading name. A single species, Trichophyton rubrum, accounts for 80% to 90% of cases.
Ringworm spreads through direct skin contact with an infected person, animal, or contaminated soil. Kids who play with cats and dogs are especially prone to picking up animal-carried strains. Sharing towels, clothing, or combs is another common route. The infection can appear on nearly any part of the body. When it affects the scalp it’s called tinea capitis, between the toes it’s athlete’s foot, and in the groin area it’s jock itch. All of these are the same type of fungus targeting different locations.
Ringworm is rarely dangerous, but it’s stubborn. Mild cases clear up with over-the-counter antifungal creams applied for several weeks. Scalp infections and widespread cases typically need prescription oral antifungal medication because creams can’t penetrate hair follicles well enough.
3. Aspergillosis
Aspergillus is a mold found virtually everywhere: in soil, compost piles, air vents, and dust. Most people breathe in its spores daily without any consequences. Problems arise when the lungs can’t clear those spores effectively.
Aspergillosis takes several forms depending on the person’s immune status and lung health. In people with asthma or cystic fibrosis, the spores can trigger an intense allergic reaction in the airways called allergic bronchopulmonary aspergillosis, which causes wheezing, coughing, and worsening breathing. In people with pre-existing lung damage from conditions like tuberculosis or COPD, the fungus can settle into a cavity in the lung and grow into a dense ball of fungal material known as an aspergilloma.
The most dangerous form is invasive aspergillosis, where the fungus spreads beyond the lungs into the blood, brain, or other organs. This primarily affects people with severely weakened immune systems: those undergoing chemotherapy, taking high-dose steroids, or recovering from stem cell or organ transplants. Invasive Aspergillus infections account for an estimated 2.1 million cases and 1.8 million deaths worldwide each year, making it the single deadliest fungal infection globally. People at high risk are often given preventive antifungal medication before they ever show symptoms.
4. Histoplasmosis
Histoplasmosis is caused by Histoplasma, a fungus that thrives in soil enriched with bird or bat droppings. It’s most common in the Ohio and Mississippi River Valleys in the United States, though it’s found across North and Central America and in parts of every other continent.
People get infected by breathing in microscopic spores disturbed from contaminated soil. Activities that stir up dirt, like demolishing old buildings, cleaning chicken coops, or exploring caves with bat colonies, carry the highest risk. Many people who inhale the spores never realize it; their immune system contains the infection without producing noticeable symptoms. When symptoms do appear, they typically look like a mild flu: fever, cough, fatigue, and body aches that resolve on their own within a few weeks.
In people with weakened immune systems, histoplasmosis can become severe. The fungus may spread from the lungs to the liver, spleen, bone marrow, or brain. This disseminated form requires months of antifungal treatment and can be fatal without it. Even in healthy individuals, breathing in a very large number of spores at once (common during demolition work or cave cleanouts) can cause acute lung inflammation serious enough to need medical care.
5. Valley Fever (Coccidioidomycosis)
Valley fever is caused by Coccidioides, a fungus that lives in the dry, alkaline soils of the southwestern United States, Mexico, and parts of Central and South America. Arizona and California’s Central Valley report the majority of U.S. cases. Like histoplasmosis, infection comes from inhaling fungal spores kicked up by wind, construction, or farming.
About 60% of people who inhale Coccidioides spores never develop symptoms at all. The other 40% typically get sick one to three weeks after exposure. Symptoms include fatigue, cough, shortness of breath, headache, night sweats, muscle aches, and sometimes a distinctive rash. The illness often mimics pneumonia closely enough that it gets misdiagnosed or treated with antibiotics that do nothing against a fungus.
Most symptomatic cases resolve without treatment over weeks to months, though the fatigue can linger. In a small percentage of cases, the fungus spreads beyond the lungs to the bones, joints, skin, or the membranes around the brain. This disseminated form is more common in people with compromised immune systems, pregnant women, and certain ethnic groups including Filipino and Black Americans for reasons that are still being studied.
How Antifungal Treatments Work
Treating fungal infections is harder than treating bacterial ones. Fungi are closer to human cells than bacteria are, which means drugs that kill fungi can also affect your own cells. The major classes of antifungal medications all target a fatty molecule called ergosterol that fungi need to build their cell membranes but humans don’t produce. Some drugs block the production of ergosterol, causing the fungal membrane to weaken and leak. Others punch holes directly in the membrane by binding to ergosterol. A newer class works differently, attacking a structural sugar in the fungal cell wall that has no equivalent in human cells.
Surface infections like ringworm and mild candidiasis usually respond to topical creams or short courses of pills. Invasive infections require stronger medications given intravenously, often for weeks or months. Antifungal resistance is a growing concern. Non-albicans Candida species, which now cause roughly two thirds of bloodstream Candida infections in the U.S., are increasingly resistant to standard treatments.
Reducing Your Risk
Prevention strategies depend on the type of fungus. For ringworm and surface candidiasis, the basics matter: keep skin clean and dry, avoid sharing personal items like towels and razors, and change out of sweaty clothes promptly. For soil-dwelling fungi like Histoplasma and Coccidioides, minimizing dust exposure is key. Wearing an N95 respirator during activities that disturb soil in endemic areas, such as digging, demolition, or spelunking, significantly lowers your risk.
For people with weakened immune systems, preventing aspergillosis and invasive candidiasis requires more aggressive measures. In hospital settings, thorough environmental disinfection, strict hand hygiene, and careful handling of shared medical equipment are the frontline defenses. Alcohol-based hand sanitizer is effective against most fungal pathogens when hands aren’t visibly dirty. People recovering from transplants or undergoing chemotherapy may be prescribed preventive antifungal medication and advised to avoid construction sites, compost, and other environments with high mold counts.