A fungal infection, or mycosis, occurs when a fungus grows in or on human tissue and causes disease or inflammation. Fungi are eukaryotic organisms, meaning their cells contain nuclei and complex internal structures. They include yeasts, moulds and organisms with more specialised growth forms. Human exposure to fungi is extremely common, and many fungal organisms exist harmlessly in the environment or as part of normal microbial communities. Disease occurs when a fungus successfully colonises susceptible tissue, invades deeper structures, produces damaging effects or proliferates when normal host controls are impaired. Fungal infections therefore range from superficial conditions affecting skin and nails to potentially life-threatening invasive infections involving internal organs.
Fungi differ biologically from bacteria and viruses in important ways. A fungal cell has a nucleus and membrane-bound organelles, and its cell membrane contains ergosterol, while its cell wall commonly contains chitin and other polysaccharides. Human cells also contain nuclei and organelles, which means there are fewer uniquely fungal structures than uniquely bacterial structures that can be targeted without affecting the host. Nevertheless, fungal biology provides useful therapeutic targets, including ergosterol synthesis and fungal cell-wall components. Antifungal drugs exploit these differences, but their therapeutic windows can be narrower than those of many antibacterial drugs because fungal cells are biologically more similar to human cells than bacteria are.
Fungal infections are often classified according to where the fungus grows and how deeply it penetrates tissue. Superficial or cutaneous infections affect the outer layers of skin, hair or nails and include dermatophyte infections such as athlete's foot and ringworm. Mucosal infections can affect areas such as the mouth or vagina, with Candida being a common cause. Subcutaneous infections can extend below the skin following traumatic implantation. Systemic or invasive fungal infections can enter deeper tissues or the bloodstream and affect organs, particularly in people with weakened immune systems. The distinction between superficial colonisation and invasive disease is clinically important because severity and treatment requirements differ dramatically.
Common fungal diseases include dermatophyte infections, candidiasis and certain mould or yeast infections affecting vulnerable people. Dermatophytes obtain nutrients from keratinised tissues and can produce scaling, itching and characteristic lesions on the skin. Candida species can normally exist on human mucosal surfaces but may overgrow under particular conditions, causing thrush or other forms of candidiasis. Invasive infections caused by organisms such as Aspergillus or Candida can be much more serious and may occur in people undergoing chemotherapy, transplantation, intensive care or other circumstances associated with impaired immunity.
Fungi can enter or colonise the human body through environmental exposure, direct contact, overgrowth of organisms already present on the body or inhalation of fungal spores. Many environmental fungi are not normally pathogenic to healthy people but can cause disease when inhaled in sufficient quantities or when host defences are compromised. Skin fungi can spread through direct contact or contaminated surfaces and personal items. Candida-related disease often represents overgrowth of organisms already present rather than acquisition from another infected person. Consequently, prevention depends on the specific fungus and route of infection rather than on one universal strategy.
Host factors strongly influence susceptibility to fungal disease. Immunosuppression, diabetes, prolonged antibiotic exposure, disruption of skin barriers, moisture and certain medical treatments can increase risk for particular fungal infections. Antibiotics can alter normal bacterial communities, sometimes allowing fungi such as Candida to proliferate. This illustrates an important ecological relationship: treatment aimed at one group of microorganisms can change the microbial environment and permit another group to expand. Severe invasive fungal infections are particularly associated with weakened immune defences, because the immune system normally prevents environmental fungi from establishing persistent or invasive growth.
Fungal infections are treated with antifungal medicines, not ordinary antibacterial antibiotics. Antifungal agents include medicines that interfere with ergosterol synthesis or function, disrupt fungal cell membranes, inhibit fungal cell-wall synthesis or interfere with other fungal-specific processes. Treatment can be topical for some skin infections, oral for selected superficial or mucosal infections, or intravenous for severe invasive disease. The appropriate drug depends on the organism, anatomical site, severity, immune status and drug susceptibility. Diagnosis may require microscopy, culture, antigen testing, molecular methods or tissue examination because fungal disease can resemble bacterial or other infections clinically.
Fungal infections are often confused with bacterial infections because both can produce inflammation, redness, pain, discharge or fever, but their causes, diagnostic methods and treatments differ. Antibiotics can sometimes increase the risk of fungal overgrowth by disrupting bacterial microbiota and therefore should not be used as treatment for fungal disease. Conversely, antifungal drugs do not generally treat bacterial infections. The clinical spectrum of fungal disease is unusually broad, extending from common superficial infections to invasive illnesses requiring hospital treatment. The key conceptual distinction is that fungi are eukaryotic microorganisms with biological structures and metabolic pathways that differ substantially from both bacteria and viruses, requiring specifically targeted antifungal therapy.
