Mycoses are infections caused by fungi, and they range from mild skin conditions that most people experience at some point to life-threatening invasive diseases that kill hundreds of thousands of people worldwide each year. The term covers everything from athlete’s foot and ringworm to deep-seated infections of the lungs, brain, and bloodstream. What makes mycoses unusual compared with bacterial or viral infections is the relative difficulty in treating them: fungi are eukaryotic organisms, meaning their cells are built much like ours, which limits the drug targets available without harming human tissue. Understanding how fungi infect us, how the body fights back, and why certain fungal threats are growing requires looking at biology, immunity, climate, and even agriculture.
How Mycoses Are Classified
Doctors and researchers group mycoses by how deep the infection goes and what kind of fungus is responsible. Superficial mycoses affect the outermost layers of skin and hair. Subcutaneous mycoses penetrate into deeper tissue, usually through a wound. Systemic mycoses involve internal organs and can be caused either by fungi that infect healthy people (endemic mycoses) or by fungi that exploit a weakened immune system (opportunistic mycoses).1PubMed Central. Classification of subcutaneous and systemic mycoses This classification matters practically because the depth of infection largely determines how aggressive treatment needs to be. A superficial dermatophyte infection might clear with a topical cream, while an invasive bloodstream infection from the same broad category of organism can require weeks of intravenous medication in an intensive care unit.
Superficial Infections and Why They Are So Common
The mycoses you are most likely to encounter personally are superficial. Dermatophytes, the fungi behind ringworm, jock itch, and athlete’s foot, are specialists at colonizing keratinized tissue like skin, nails, and hair. They produce enzymes that break down keratin and other structural proteins, which is how they feed and spread across the skin surface. Research on dermatophyte virulence has confirmed that all tested species produce phospholipase, lipase, protease, and gelatinase, a toolkit that lets them digest multiple components of human tissue.2PubMed Central. Enzymatic and Non-Enzymatic Virulence Activities of Dermatophytes on Solid Media These infections are rarely dangerous in otherwise healthy people, but they are remarkably persistent. Toenail fungus, for example, can take months of oral antifungal therapy to resolve and frequently recurs.
Another common superficial mycosis is pityriasis versicolor, caused by Malassezia yeasts that naturally live on everyone’s skin. When conditions favor overgrowth, typically warm, humid weather combined with oily skin, they produce discolored patches. The infection is cosmetically annoying but medically benign, and it illustrates a recurring theme in mycology: many fungi that cause disease in humans are already living on or inside us as commensals, tipping into pathogenicity only when the local environment shifts.
Why Body Temperature Matters More Than You Think
One of the most elegant defenses mammals have against fungi has nothing to do with the immune system in the traditional sense. It is body heat. Most environmental fungi grow well at ambient temperatures but cannot tolerate the sustained warmth of a mammalian body. Modeling the tradeoff between the metabolic cost of maintaining a high body temperature and the benefit of excluding fungal pathogens, researchers found that the optimal temperature balancing those two pressures lands at about 36.7°C, almost exactly the human core temperature.3PubMed Central. Mammalian endothermy optimally restricts fungi and metabolic costs This thermal barrier is thought to be one reason invasive fungal diseases are relatively rare in healthy mammals compared with other infections.4The Journal of Infectious Diseases. Vertebrate Endothermy Restricts Most Fungi as Potential Pathogens
The flip side is revealing. Cold-blooded animals lack this thermal shield and suffer devastating fungal diseases. Amphibian chytridiomycosis, caused by the fungus Batrachochytrium dendrobatidis, has driven population crashes and extinctions across entire continents. White-nose syndrome in bats similarly exploits hibernation: the fungus grows on the cold wing and nose tissue of bats whose body temperatures drop during torpor. Fungal emerging diseases of wildlife appear to be increasing faster than those caused by bacteria, viruses, or protozoa, and their common thread is that they tend to strike either cold-blooded hosts or the colder extremities of warm-blooded ones.5PubMed Central. Disentangling host, pathogen, and environmental determinants of a recently emerged wildlife disease: lessons from the first 15 years of amphibian chytridiomycosis research
The Immune System’s Fungal Sensors
Beyond body temperature, your immune system has specialized molecular machinery for detecting fungi. The receptor called Dectin-1, found on the surface of macrophages and other immune cells, recognizes beta-glucan, a sugar polymer that is a major structural component of fungal cell walls. When Dectin-1 latches onto beta-glucan, it triggers inflammation and phagocytosis, the process by which immune cells engulf and destroy invaders. Research has shown that the physical structure of beta-glucan matters: highly structured glucan molecules drive Dectin-1 receptors to cluster together on the cell surface, and this clustering is what switches on downstream signaling.6Biophysical Journal. Dectin-1A senses β-glucan quaternary structure via multivalent ligand-receptor interactions Dectin-1 also partners with another protein, galectin-3, inside macrophages to mount a full inflammatory response. When this partnership breaks down in experimental models, the immune response to Candida falls apart.7PubMed Central. Fungal recognition is mediated by the association of dectin-1 and galectin-3 in macrophages
Fungi, of course, have evolved countermeasures. Candida albicans, the most common cause of mucosal and bloodstream yeast infections, can alter its cell wall during infection so that the beta-glucan layer is masked by an outer mannan coat, reducing Dectin-1-dependent recognition. Different strains vary in how much they remodel their walls in vivo, and this variation changes which immune pathways the host relies on to control the infection. Strains with higher chitin content in their cell walls show reduced dependence on Dectin-1 for immune control, effectively slipping past one of the host’s primary detection systems.8PLOS Pathogens. Differential Adaptation of Candida albicans In Vivo Modulates Immune Recognition by Dectin-1
A second critical arm of antifungal immunity involves interleukin-17 (IL-17), a signaling molecule produced by a subset of immune cells called Th17 cells. IL-17 plays a surprisingly specific role in defending against Candida at mucosal surfaces like the mouth, gut, and reproductive tract.9PubMed Central. IL-17-Mediated Immunity to the Opportunistic Fungal Pathogen Candida albicans People who lack functional IL-17 signaling, whether due to genetic conditions or immunosuppressive drugs, are highly susceptible to chronic mucosal candidiasis. Mouse studies have confirmed that Th17 cells promote faster fungal clearance and confer long-term protection against oral Candida infections.10PubMed Central. Th17 cells confer long-term adaptive immunity to oral mucosal Candida albicans infections
How Dimorphic Fungi Cause Systemic Disease in Healthy People
Most fungi that cause serious systemic infection in people with normal immune systems share a trick: they are thermally dimorphic. In the environment at cooler temperatures, they grow as molds, producing spores that become airborne. Once inhaled into the warm lungs at 37°C, they shift shape into a yeast form, and this switch is essential for their ability to cause disease.11PubMed Central. Fungal Dimorphism and Virulence: Molecular Mechanisms for Temperature Adaptation, Immune Evasion, and In Vivo Survival The classic endemic mycoses, including histoplasmosis, blastomycosis, and coccidioidomycosis (Valley fever), all follow this pattern. They are restricted to particular geographic regions because their mold forms require specific soil and climate conditions, but within those regions, exposure is common and most infections are mild or asymptomatic.
Valley fever has attracted particular attention because its geographic range is shifting. The fungus Coccidioides thrives in hot, arid soils, and climate modeling predicts that as the western United States gets warmer and drier, the area suitable for the fungus could more than double by 2100 under high-warming scenarios, with new endemic states potentially including Idaho, Wyoming, Montana, and the Dakotas, and cases increasing by about half.12PubMed Central. Expansion of Coccidioidomycosis Endemic Regions in the United States in Response to Climate Change A more recent modeling study estimated that by 2050, areas newly suitable for Coccidioides could contain roughly 23 million people who currently live outside the endemic zone, rising to about 30 million by 2100.13PubMed. Environmental suitability of Coccidioides in the USA under climate change scenarios: a modelling study Both studies also flagged Texas as a state where the fungus may already be underrecognized because coccidioidomycosis is not a reportable disease there.
Opportunistic Mycoses and Who Is Most at Risk
For people with healthy immune systems, opportunistic fungi are generally harmless commensals or environmental organisms that the body clears without difficulty. The picture changes dramatically in immunocompromised individuals. People undergoing chemotherapy, organ transplant recipients on immunosuppressive drugs, and those with advanced HIV are all vulnerable to invasive fungal infections. The depth and duration of neutropenia, the depletion of infection-fighting white blood cells, are among the strongest predictors of risk, with Candida and Aspergillus species being the most frequent culprits.14PubMed. Prevention, diagnosis, and treatment of invasive fungal infections in patients with cancer and neutropenia
Cryptococcus neoformans, which causes cryptococcal meningitis, has a particularly unsettling method of reaching the brain. It hitches a ride inside macrophages, the very immune cells sent to kill it, and is carried across the blood-brain barrier in what researchers call a “Trojan horse” mechanism. Imaging studies have captured fungus-laden immune cells crossing the barrier through transendothelial pores, and this method even allows fungal mutants that cannot cross the barrier on their own to reach the central nervous system.15PubMed Central. Trojan Horse Transit Contributes to Blood-Brain Barrier Crossing of a Eukaryotic Pathogen
Mucormycosis, sometimes called black fungus, is another feared opportunistic infection. It is caused by molds in the order Mucorales and has a strong association with uncontrolled diabetes, where high blood sugar and acidic blood create a favorable environment for fungal growth. In a 21-year study from southern Iran, diabetes was the second most common predisposing factor for mucormycosis after leukemia, and the overall mortality rate among diabetic patients with mucormycosis exceeded 53%.16PubMed Central. Predisposing Factors for Mucormycosis in Patients with Diabetes Mellitus; An Experience of 21 Years in Southern Iran The disease gained widespread public attention during COVID-19 waves in India, where steroid use in patients with pre-existing diabetes created a surge in cases.
When Flu Opens the Door to Aspergillus
Invasive aspergillosis has traditionally been thought of as a disease of the profoundly immunocompromised, but severe influenza can create the conditions for it even in people without classic risk factors. Influenza damages the airway lining, disrupts normal clearance of inhaled particles, and impairs the function of alveolar macrophages. It also shifts the immune balance systemically, causing lymphopenia and suppressing Th1 responses, both of which are known risk factors for aspergillosis.17PubMed Central. Invasive Aspergillosis Associated With Severe Influenza Infections
More recent work has identified a specific mechanism: severe influenza triggers an exaggerated interferon-gamma response that, paradoxically, cripples the very immune pathways needed to fight Aspergillus. The excess interferon-gamma depletes macrophages, suppresses the Th17 response critical for fungal defense, and inhibits a specialized killing process inside macrophages. In mouse models, reducing or eliminating interferon-gamma production fully protected animals from invasive fungal infection following influenza.18The Lancet Microbe. Unravelling the immunopathogenesis of influenza-associated pulmonary aspergillosis to identify new therapeutic targets This finding hints at possible therapeutic strategies for influenza-associated pulmonary aspergillosis, a condition that was recognized increasingly during COVID-19 ICU admissions as well.
Biofilms and Why Fungal Infections Recur
One reason certain mycoses are so stubborn is biofilm formation. Candida species, in particular, readily form biofilms on medical devices like catheters, prosthetic joints, and heart valves. A biofilm is a structured community of cells embedded in a self-produced matrix that sticks to a surface. The matrix physically shields the fungi from both antifungal drugs and immune cells, making biofilm-associated infections extremely difficult to treat and a major driver of recurrent disease.19PubMed Central. Fungal biofilms, drug resistance, and recurrent infection Mature Candida biofilms are far more resistant to antifungal agents than the same species growing as free-floating cells.20PubMed Central. Resistance of Candida albicans Biofilms to Drugs and the Host Immune System In many cases, the only effective treatment is to physically remove the colonized device, which is not always possible.
Candida species including the relatively recently described Candida dubliniensis have become major agents of hospital-acquired infections, and many of these infections are associated with biofilm formation on intravascular catheters.21Emerging Infectious Diseases. Fungal Biofilms and Drug Resistance The resistance problem with biofilms involves multiple mechanisms acting in concert: the matrix limits drug penetration, cells within the biofilm grow more slowly (and many antifungals work best against actively dividing cells), and the biofilm environment favors the expression of efflux pumps that actively push drugs back out of cells.22PubMed Central. Mechanisms of Antifungal Drug Resistance
The Rise of Candida auris
No discussion of modern mycoses is complete without Candida auris, a species first identified in 2009 that has since caused outbreaks in hospitals on every inhabited continent. What makes it alarming is a combination of traits rarely seen together in a single fungal pathogen: it persists on hospital surfaces for weeks, spreads readily from patient to patient, is difficult to identify with standard laboratory methods, and is frequently resistant to multiple classes of antifungal drugs.23PubMed Central. Candida auris, an Agent of Hospital-Associated Outbreaks: Which Challenging Issues Do We Need to Have in Mind? Genomic analysis of UK isolates found that every isolate carried mutations in a gene linked to fluconazole resistance, and these mutations were specific to different genetic lineages of the fungus, suggesting the resistance evolved independently multiple times or was present in the founding strains before they arrived.24npj Antimicrobials and Resistance. Genomic epidemiology describes introduction and outbreaks of antifungal drug-resistant Candida auris C. auris has been classified as an urgent public health threat by the CDC, and infection-control measures in affected hospitals resemble those used for the most dangerous antibiotic-resistant bacteria.
Agricultural Fungicides and Cross-Resistance
An under-recognized factor in antifungal resistance connects the farm field to the hospital ward. Triazole antifungals are the backbone of both clinical treatment for invasive aspergillosis and crop protection in agriculture. Certain strains of Aspergillus fumigatus found infecting humans carry resistance mutations that match patterns seen in environmental isolates exposed to agricultural fungicides rather than clinical drugs.25PubMed Central. Trends in Agricultural Triazole Fungicide Use in the United States, 1992-2016 and Possible Implications for Antifungal-Resistant Fungi in Human Disease The concern is that widespread spraying of triazoles on crops selects for resistant Aspergillus in the soil, and people then inhale those resistant spores.26PubMed Central. Global Aspects of Triazole Resistance in Aspergillus fumigatus with Focus on Latin American Countries
The problem extends beyond Aspergillus. Laboratory experiments with Candida tropicalis, another cause of human bloodstream infections, showed that exposing the fungus to the agricultural triazole tebuconazole drove the rapid evolution of resistance not only to tebuconazole itself but also to clinical azoles including fluconazole, voriconazole, itraconazole, and posaconazole. Some evolved isolates reached resistance levels hundreds of times higher than the starting strain.27PLoS Biology. An agricultural triazole induces genomic instability and haploid cell formation in the human fungal pathogen Candida tropicalis This kind of cross-resistance is the worst-case scenario for a “One Health” problem, where actions in agriculture directly undermine the drugs available to treat human infections.
Diagnosing Invasive Mycoses
Diagnosing deep fungal infections remains one of the harder problems in infectious disease. Fungi grow slowly in culture, and by the time blood cultures turn positive, the infection may already be advanced. Two blood-based tests have become important adjuncts: the beta-D-glucan (BDG) test, which detects a component of most fungal cell walls, and the galactomannan (GM) test, which is more specific to Aspergillus. In patients with blood cancers, combining the two tests into a composite score significantly improved diagnostic accuracy compared with either test alone, achieving about 73% sensitivity and 87% specificity in one recent evaluation.28PubMed Central. Assessment of the 1,3-beta-D-glucan test and the galactomannan antigen test in the detection of invasive fungal infections in patients with hematological diseases
These tests have limitations, though. In children and young adults with leukemia who were already receiving antifungal prophylaxis, both the BDG and GM assays failed to detect any true positive cases; the preventive medication suppressed fungal growth enough to keep biomarker levels below the detection threshold.29PubMed Central. Prospective Evaluation of Galactomannan and (1→3) β-d-Glucan Assays as Diagnostic Tools for Invasive Fungal Disease in Children, Adolescents, and Young Adults With Acute Myeloid Leukemia Receiving Fungal Prophylaxis That study’s authors recommended against using these assays for routine surveillance in patients already on prophylaxis, a practical caution that matters for hospitals making testing protocols. Newer molecular approaches like PCR-based detection are filling some of these gaps, but no single test yet catches all invasive mycoses reliably.
The Mycobiome, Your Resident Fungi
Just as your gut hosts trillions of bacteria, it also harbors a community of fungi called the mycobiome. Different body sites carry distinct fungal populations, and the composition of these communities is associated with various disease states.30PubMed Central. The human mycobiome in health and disease The gut mycobiome is numerically small compared with the bacterial population, but it punches above its weight. Intestinal fungi help regulate immune balance, influence the assembly of bacterial communities sharing the same space, and contribute to both normal physiology and disease progression when the balance tips.31The Lancet Infectious Diseases. Gut mycobiota: an underappreciated component of the human microbiome
This raises questions that researchers are only beginning to answer. Does antibiotic use, which disrupts bacterial communities, also shift the mycobiome in ways that promote fungal overgrowth? Do probiotics containing fungi (like Saccharomyces boulardii, already used for antibiotic-associated diarrhea) work partly by reshaping the mycobiome? Can changes in the fungal community serve as early markers of conditions like inflammatory bowel disease, where mycobiome shifts have been observed? The field is still young, but it is changing how infectious disease specialists think about the line between fungal commensalism and disease.
Why There Are No Approved Fungal Vaccines
Despite the enormous burden of fungal disease, there is currently no licensed vaccine against any human mycosis. This is not for lack of trying. The challenge is partly biological: fungi are eukaryotes with complex and adaptable surfaces, and the immune responses needed to control them vary depending on the species and the body site involved. It is also partly economic. The populations at highest risk for invasive mycoses, immunocompromised patients and people in tropical low-income settings, represent difficult markets for vaccine development. Recent reviews have highlighted strategies that may accelerate progress, including computational approaches to identify the best target molecules, nanoparticle-based delivery systems, and combining vaccines with probiotics, but no candidate has yet reached late-stage clinical trials.32PubMed Central. Advances in fungal vaccine development: Progress in the face of emerging challenges For now, antifungal drugs and prophylaxis remain the primary tools, which makes the creeping problem of resistance all the more consequential.