Fungi are eukaryotic organisms that include yeasts, molds, and mushrooms. They are distinct from both plants and bacteria, possessing cell walls made of chitin. Unlike plants, fungi lack chlorophyll and do not perform photosynthesis; instead, they are heterotrophs, absorbing nutrients from their surroundings after secreting digestive enzymes. This group interacts with humans in complex ways, encompassing both disease-causing pathogens and sources of benefits for health and industry.
Fungal Infections and Disease
Diseases caused by fungal growth in human tissue are known as mycoses. These infections are broadly categorized based on the depth of tissue invasion, ranging from minor surface issues to life-threatening internal disease. Fungi often enter the body through the inhalation of airborne spores or via breaks in the skin barrier.
The most common human fungal diseases are superficial mycoses, confined to the skin’s outermost layer, hair, and nails. These are frequently caused by dermatophytes, a group of fungi that require keratin for growth, resulting in conditions often referred to as ringworm or tinea. Examples include tinea pedis (athlete’s foot) and tinea unguium (nail infections).
Subcutaneous mycoses occur when fungi breach the skin and infect the deeper dermis, subcutaneous tissue, and sometimes bone. These infections are typically acquired through a traumatic injury, such as a scratch from a thorn or splinter, which introduces the fungus from the environment. Sporotrichosis, sometimes called “rose gardener’s disease,” is a well-known example.
The most serious are systemic mycoses, which affect internal organs and can disseminate throughout the body. These infections often originate in the lungs after inhaling spores from environmental fungi, such as Histoplasma or Coccidioides. They are especially concerning for immunocompromised individuals, where common fungi can become opportunistic pathogens. Invasive infections caused by fungi like Aspergillus or Candida are frequently seen in patients with weakened immune systems due to conditions like HIV, chemotherapy, or organ transplantation.
Exposure Risks From Spores and Toxins
Beyond active infection, fungi can cause harm through exposure to their airborne components or their toxic chemical byproducts. Fungal spores, which are constantly present in the air, act as potent allergens for many people. Inhaling these microscopic particles can trigger allergic responses, including respiratory symptoms like nasal congestion, sneezing, and irritation.
Exposure to high concentrations of airborne mold spores can exacerbate existing conditions, such as asthma, for sensitive individuals. This reaction is an immune response to the mold components, rather than an infection, leading to symptoms like wheezing and difficulty breathing. It can occur even if the spores are dead, as the body reacts to the cellular fragments.
Certain molds produce mycotoxins, chemical byproducts that can contaminate food and pose significant health risks. These toxins, such as aflatoxins, are often found in improperly stored agricultural products like grains, nuts, and spices. Ingestion of contaminated food is the primary route of exposure, and long-term consumption has been associated with severe health consequences, including immune suppression and liver damage. Mycotoxins are chemically stable compounds and can persist in food even after the mold itself has been killed.
Beneficial Contributions to Health and Nutrition
Fungi have yielded some of the most important discoveries in modern medicine, largely through their production of bioactive compounds. The most famous example is penicillin, derived from the mold Penicillium rubens. This fungal compound revolutionized the treatment of bacterial infections by interfering with the microbes’ ability to build cell walls.
Fungi are also the source of other pharmaceuticals, including immunosuppressants used in organ transplantation and statins for lowering cholesterol. The drug cyclosporin, which prevents the body from rejecting transplanted organs, was originally isolated from the fungus Tolypocladium inflatum. Meanwhile, statins, which help manage cardiovascular risk, have their origins in compounds produced by fungi like Aspergillus terreus.
As a food source, edible mushrooms offer substantial nutritional value. They are a good source of protein, dietary fiber, and various vitamins and minerals. Notably, mushrooms can provide Vitamin D when exposed to UV light, and they are rich in B vitamins. Fungal yeasts, such as Saccharomyces cerevisiae, are also indispensable in food production, being responsible for the leavening of bread and the fermentation of alcoholic beverages.
Fungi as Components of the Human Microbiome
The human body naturally hosts a community of fungi, collectively known as the mycobiome. These fungal residents coexist with a much larger and more diverse population of bacteria on the skin, in the oral cavity, and throughout the gastrointestinal tract. In a healthy state, this fungal community maintains a stable, symbiotic balance with the host and the bacterial populations.
This coexistence is largely harmless, with the fungi often playing a role in regulating immune responses. However, this delicate balance can be easily disrupted, a state known as dysbiosis. A common cause is the use of broad-spectrum antibiotics, which eliminate competing bacteria, allowing certain fungi to overgrow opportunistically.
For example, the yeast Candida albicans is a common commensal in the gut and on mucosal surfaces. When the bacterial population is suppressed, Candida can proliferate rapidly, leading to symptomatic overgrowth like oral or vaginal candidiasis. This highlights the complex, dynamic interplay between the fungal and bacterial communities necessary for maintaining human health.
Prevention and Management of Fungal Exposure
Managing fungal risks involves both environmental control and basic hygiene practices. Since most molds thrive in damp environments, controlling moisture is the most effective way to prevent harmful fungal growth in indoor spaces. Keeping the relative humidity below 70% is a general guideline to prevent the activation and proliferation of most mold spores.
Prevention of mycotoxin contamination requires careful management of agricultural products, especially grains and nuts. Grains must be dried quickly after harvest to a safe moisture level—often 14% to 15%—to inhibit the growth of storage fungi like Aspergillus and Penicillium. Proper storage involves maintaining a cool environment, preventing insect infestation, and avoiding cross-contamination between old and new batches of food.
When fungal infections occur, they are typically managed with antifungal medications, which work by targeting unique fungal components, such as the cell membrane. For persistent or systemic infections, diagnostic methods, such as culturing or molecular tests, are used to identify the specific fungal species. Simple hygiene, such as keeping skin clean and dry and avoiding sharing items that contact skin, can help prevent the spread of superficial infections like athlete’s foot.