Fungal infections, known medically as mycoses, are diseases caused by fungi, which are a group of organisms distinct from bacteria and viruses. Fungi are eukaryotes, meaning their cells possess a true nucleus and complex internal structures. This cellular complexity is why fungal infections are treated with specific antifungal medications, as antibiotics effective against bacteria have no effect on fungal cells. Millions of fungal species exist, but only a few hundred cause human disease. Most fungi that cause human infection are either opportunistic, attacking only when the body’s defenses are weakened, or true pathogens capable of causing disease in a healthy host.
Categorizing Fungal Infections by Location
The classification of mycoses is based on the depth of tissue penetration and the location of the infection.
The most common and least severe types are superficial mycoses, confined to the outermost layer of the skin, the hair shaft, and the nail plate. These infections are largely cosmetic and typically elicit no noticeable immune response or inflammation.
Cutaneous mycoses involve the entire epidermis, including the hair and nails, where the fungi consume the protein keratin. These infections often cause inflammation characterized by scaling, redness, and itching, such as with athlete’s foot or ringworm. These conditions remain limited to the skin layers.
Subcutaneous mycoses represent a more serious category, penetrating below the skin surface into the subcutaneous tissue, fascia, and sometimes bone. These infections usually begin after a traumatic injury, such as a thorn prick, introduces the fungus directly into the deeper layers. They often manifest as localized, chronic ulcers, nodules, or abscesses at the site of inoculation.
The most serious category is systemic mycoses, which affect internal organs and spread throughout the body. These infections often start in the lungs after fungal spores are inhaled from the environment. From the lungs, the fungi can disseminate through the bloodstream to affect organs like the brain, kidneys, and liver, posing a significant threat to life.
Major Pathogens and the Diseases They Cause
Dermatophytes are responsible for nearly all cutaneous mycoses, including infections commonly known as tinea. These filamentous fungi, which include species from the genera Microsporum, Trichophyton, and Epidermophyton, require keratin to grow. They cause ringworm, jock itch, and nail fungus, and are highly contagious, spreading through contact with infected skin scales or contaminated surfaces.
The yeast Candida is a major opportunistic pathogen, with Candida albicans being the most frequent species involved in human disease. This fungus is a normal part of the human microbiome, living harmlessly on the skin and mucosal surfaces. However, overgrowth can lead to localized infections like oral thrush or vaginal yeast infections. In severely ill patients, it can cause invasive candidiasis, a deep-seated infection of the bloodstream or organs.
Molds of the genus Aspergillus, especially Aspergillus fumigatus, are ubiquitous and acquired through the inhalation of airborne spores. While healthy individuals clear the spores without issue, immunocompromised patients can develop pulmonary aspergillosis, a serious lung infection. These fungi grow as branching filaments and can form fungal balls within the lung cavities.
Dimorphic fungi are true pathogens that can cause disease even in people with healthy immune systems. They have the unique ability to change their form depending on the temperature. In the environment, they exist as a mold, but once inhaled and inside the body, they convert into a yeast-like form. Examples include Histoplasma capsulatum (histoplasmosis) and Coccidioides immitis (coccidioidomycosis or Valley Fever), both of which start as lung infections.
Factors Determining Host Susceptibility
The integrity of the host’s immune system is the primary factor determining susceptibility to most fungal infections, particularly those caused by opportunistic species. Individuals with compromised immune function, such as those undergoing chemotherapy, receiving organ transplants, or living with HIV, have a substantially increased risk of developing invasive mycoses. These conditions directly impair the function of immune cells, which are the body’s main defense against fungi.
Disruption of the natural microbial balance, often caused by prolonged antibiotic use, also increases the risk of fungal overgrowth. Antibiotics eliminate competitive bacteria that normally keep commensal fungi, like Candida, in check. This ecological imbalance allows the fungus to proliferate and potentially invade tissue barriers, often leading to mucosal yeast infections.
Underlying chronic diseases, such as uncontrolled diabetes mellitus, create a metabolic environment that favors fungal growth. High blood glucose levels impair immune cell function and provide a nutrient-rich environment for certain fungi to thrive. Breaches in physical barriers, such as surgical wounds, indwelling medical devices, and severe trauma, offer direct entry points for fungi into the bloodstream and deep tissues.
Genetic variations in immune system genes can also predispose certain individuals to specific fungal infections. Polymorphisms in genes that code for pattern recognition receptors can impair the body’s ability to recognize and mount an effective initial immune response against fungal components. This genetic susceptibility highlights the complex interplay between the pathogen and the host’s specific biological defenses.
Diagnosis and Management Approaches
Diagnosing a fungal infection requires a combination of clinical suspicion and laboratory confirmation, as symptoms can often mimic those of bacterial or viral diseases. The traditional gold standard for identification involves microscopic examination of tissue or fluid samples treated with potassium hydroxide to visualize fungal elements. Culturing the organism from the sample on specialized media remains a primary method to identify the specific pathogen, although this process can be slow.
More rapid diagnostic techniques are increasingly utilized for systemic infections, including serological tests that detect fungal antigens or antibodies in the patient’s blood. Molecular assays, such as nucleic acid amplification tests, can quickly identify the fungus’s genetic material, which is useful for fast-growing molds like Aspergillus. Imaging studies, including CT scans and MRIs, also play a role in visualizing the extent of deep organ involvement, particularly in the lungs and brain.
The management of mycoses relies on antifungal medications, which are categorized into several classes based on their mechanism of action. Superficial infections are typically treated with topical formulations like creams or ointments containing azoles or allylamines. Systemic mycoses require more potent, often intravenous, agents such as polyenes, echinocandins, or triazoles, which work throughout the body.
The choice of medication and the duration of treatment depend on the identified species of fungus, the site of the infection, and the patient’s immune status. Treatment for deep-seated infections is frequently prolonged, lasting several weeks to months, and may involve surgical debridement to remove infected tissue. Early and accurate diagnosis is paramount for initiating appropriate therapy, improving the outcome for patients with systemic mycoses.