Fungal diseases, or mycoses, occur when fungi infect the human body. Most fungi live harmlessly in the environment, and some normally exist on human skin or in the body without causing illness. Disease develops when a fungus grows in a place where it should not, multiplies excessively, or takes advantage of weakened defenses.
Fungal infections range from minor skin conditions, such as athlete’s foot, to serious infections that can spread through the lungs, bloodstream, or internal organs. The severity depends on the type of fungus, where it enters the body, how far it spreads, and the person’s immune defenses.
How fungi cause disease
Fungi are organisms distinct from bacteria, viruses, and parasites. Human fungal infections are caused by several different groups of fungi, including yeasts and molds.
Unlike many bacteria, fungi are relatively similar to human cells in their basic biology. This matters because medicines have fewer fungal-specific targets that can be attacked without also affecting human cells. As a result, some fungal infections can be difficult to treat, particularly when they occur deep inside the body.
Fungi can cause disease in several ways. A fungus may grow on the outer layers of skin and feed on keratin, a tough protein found in skin, hair, and nails. Other fungi can enter through the lungs after their spores are inhaled. Some yeasts that normally live on the body can multiply excessively when local conditions change. In people with severely impaired immunity, certain fungi can invade tissues and spread to organs that are normally protected from infection.
The body’s response to the fungus can also contribute to illness. Inflammation triggered by an infection may cause swelling, irritation, tissue damage, or impaired organ function.
The main types of fungal infections
Fungal diseases are often classified according to how deeply they involve the body.
Superficial and skin infections
These are among the most common fungal infections. Dermatophytes, a group of fungi that grow in keratin-containing tissues, cause infections commonly known as ringworm. Despite its name, ringworm is not caused by a worm.
Depending on the affected area, these infections include athlete’s foot, jock itch, scalp ringworm, and fungal infections of the skin or nails. They can produce itching, scaling, cracking, redness, or changes in the appearance and thickness of nails. Scalp infections can sometimes cause hair loss and inflammation.
Warm, moist environments can favor fungal growth. Shared surfaces, footwear, towels, and direct skin contact can sometimes facilitate transmission, depending on the particular infection.
Mucosal and yeast infections
Some fungi normally live on the human body without causing problems. Candida, a group of yeasts, is one example. It can be found on the skin and in areas such as the mouth and gastrointestinal tract.
When the balance between the organism and its host changes, Candida can overgrow and cause candidiasis. Depending on the location, this can produce conditions such as oral thrush or vaginal yeast infections.
The presence of Candida does not necessarily mean that an infection exists. Diagnosis depends on the symptoms, affected site, and clinical findings rather than simply detecting the organism.
Lung infections
Some fungi exist in soil or other environmental sources and release microscopic spores into the air. People can inhale these spores without becoming ill, but certain fungi can cause respiratory infections.
Fungal lung infections may cause cough, fever, chest discomfort, fatigue, or shortness of breath. Some infections remain limited to the lungs, while others can spread beyond them, particularly in people whose immune systems cannot effectively contain the fungus.
A person’s location, occupation, environmental exposures, and immune status can influence the likelihood of encountering particular environmental fungi.
Invasive fungal infections
The most dangerous fungal diseases occur when fungi invade tissues or enter the bloodstream and reach internal organs. These infections are called invasive fungal infections.
They are much more likely in people with severely weakened immune defenses, including some people receiving cancer treatment, undergoing organ transplantation, or taking medications that strongly suppress immunity. Severe illness and certain medical procedures can also increase susceptibility.
Invasive infections can affect the lungs, brain, bloodstream, kidneys, or other organs. Because they can progress rapidly and produce nonspecific symptoms, recognizing them can be difficult. They generally require prompt medical evaluation and treatment with systemic antifungal medication.
Why some people are more vulnerable
A healthy immune system provides several layers of protection against fungi. Physical barriers such as intact skin and mucous membranes prevent many organisms from entering the body. The immune system can then recognize and eliminate fungi that cross those barriers.
Certain circumstances weaken these defenses or create opportunities for fungal growth.
Risk can increase with:
- A weakened immune system
- Cancer and some cancer treatments
- Organ or stem-cell transplantation
- Medications that suppress immune activity
- Advanced HIV infection
- Prolonged hospitalization or critical illness
- Certain chronic medical conditions
- Damage to the skin or mucous membranes
- Recent or prolonged use of antibiotics in situations where they disrupt normal microbial communities
Risk is not determined by immune status alone. Exposure also matters. Someone can encounter a fungus in soil, decaying vegetation, buildings, or other environments without becoming infected, while a susceptible person exposed to the same organism may develop disease.
How fungal infections spread
There is no single way that fungal diseases are transmitted.
Some spread through direct contact or contaminated objects. This is common with certain skin infections. Others are acquired from the environment when a person inhales fungal spores. Some infections arise from organisms that are already living on the person’s body rather than being caught from another person.
This distinction is important because not every fungal infection is contagious. A vaginal yeast infection, for example, generally results from an overgrowth of Candida already present in or on the body rather than from catching an infection from another person.
Fungi can also enter through breaks in the skin. In people with impaired immunity, an infection that begins at one site may have the opportunity to spread internally.
What symptoms can fungal diseases cause?
Symptoms depend heavily on the location and type of infection.
Skin infections commonly cause itching, scaling, redness, cracking, or changes in the nails. Oral Candida infections may produce creamy or white patches and soreness in the mouth. Vaginal candidiasis can cause itching, irritation, and abnormal discharge.
Respiratory fungal infections can resemble other lung illnesses, producing symptoms such as cough, fever, fatigue, chest pain, or difficulty breathing. Invasive infections may cause fever and other signs of serious illness, but their symptoms can vary considerably depending on which organs are involved.
Because many fungal infections resemble bacterial or viral illnesses, symptoms alone often cannot identify the cause.
How fungal diseases are diagnosed
Diagnosis begins with the person’s symptoms, medical history, immune status, and possible environmental or occupational exposures. Clinicians may examine affected skin or nails and collect samples for laboratory testing.
Depending on the suspected infection, testing may involve microscopic examination, fungal culture, detection of fungal components, molecular testing, blood tests, or examination of tissue.
For deep or invasive infections, imaging may be used to look for abnormalities in the lungs or other organs, sometimes together with tissue or fluid samples. Identifying the specific fungus can be particularly important because different fungi respond differently to antifungal drugs.
A laboratory finding must also be interpreted in context. Some fungi or yeasts can be present as normal colonizers or contaminants rather than being the cause of a person’s symptoms.
How fungal infections are treated
Treatment depends on the organism, location, and severity of the infection.
Topical antifungal medicines are applied directly to the skin or another affected surface and are commonly used for localized infections. More extensive or difficult infections may require systemic antifungal medicines, which travel through the body.
Several major classes of antifungal drugs exist, including azoles, polyenes, and echinocandins. They work through different mechanisms, such as interfering with fungal cell membranes or disrupting components needed for fungal growth.
Treatment can be more complicated than treatment for many common bacterial infections. Antifungal drugs can interact with other medications and may cause significant side effects, particularly when used for serious systemic infections. Some fungi can also develop antifungal resistance, meaning that a drug that would normally work becomes less effective.
For this reason, the appropriate treatment depends on the specific infection rather than simply choosing a general-purpose antifungal drug.
Why fungal diseases can be difficult to treat
One of the fundamental challenges of antifungal therapy is that fungi are eukaryotes, meaning their cells share many features with human cells. Bacteria have several structures and biological processes that differ substantially from those of humans, providing useful targets for antibiotics. Fungi offer fewer such differences.
Antifungal drugs therefore have to exploit vulnerabilities that fungi possess but humans either lack or use differently. This limits the number of available drug targets and can create a narrower margin between treating the infection and causing toxicity.
Drug penetration is another consideration. An antifungal that reaches effective concentrations in the bloodstream may not reach the same concentration in every tissue. The location of the infection can therefore influence both drug selection and treatment duration.
How to reduce the risk of fungal disease
Many everyday fungal infections can be prevented by reducing moisture, avoiding unnecessary exposure to infected skin or contaminated personal items, and maintaining healthy skin barriers.
Keeping feet dry, changing damp clothing, avoiding the sharing of towels or footwear, and practicing good skin and nail hygiene can help reduce the risk of some superficial infections. Prompt attention to persistent skin or nail changes can also prevent an infection from becoming more extensive.
Environmental fungal infections are harder to prevent because fungal spores are widespread. People with severely weakened immune systems may receive specific advice from their healthcare teams about avoiding particular environmental exposures, depending on their medical circumstances.
Good general health practices do not eliminate the possibility of fungal infection, and people with weakened immunity may develop fungal disease despite taking appropriate precautions.
When a fungal infection needs medical attention
A mild, localized skin infection is usually very different from an invasive fungal disease. Medical evaluation is especially important when an infection is persistent, spreading, unusually severe, repeatedly returning, or affecting the scalp or nails.
Urgent evaluation is warranted for serious symptoms such as difficulty breathing, confusion, severe weakness, or other signs of a rapidly worsening illness—particularly in someone with a weakened immune system.
The central distinction is that fungi are not inherently harmful organisms. Humans routinely encounter them, and some live normally on or in the body. Disease occurs when the relationship between fungus and host changes—through excessive growth, tissue invasion, environmental exposure, or impaired defenses. Understanding that distinction explains why fungal diseases range from minor surface infections to potentially life-threatening illnesses.
