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Overview of Fungal Infections


Paschalis Vergidis

, MD, MSc, Mayo Clinic College of Medicine & Science

Reviewed/Revised Nov 2023
Topic Resources

Fungi are living organisms, but they are not plants or animals. All living things are divided into categories called kingdoms, and fungi have their own kingdom.

Some fungi cause infections in people:

Fungi can grow in two forms:

  • Yeasts: Single round cells

  • Molds: Many cells forming long, thin threads called hyphae

Some fungi exist as both forms during their life cycle. These fungi are called dimorphic fungi.

Fungi often grow in soil and decaying plant material. Many fungi, including bread molds and mushrooms, can be seen with the naked eye.

Did You Know...

  • Fungi are classified as their own kingdom of living organisms. They are neither plants nor animals.

Fungi reproduce by spreading microscopic spores. These spores are often present in the air and soil, where they can be inhaled or come into contact with the surfaces of the body, primarily the skin. Consequently, fungal infections usually begin in the lungs or on the skin.

Of the wide variety of spores that land on the skin or are inhaled into the lungs, most do not cause infection. A few types cause infection only in people who have one of the following:

  • A weakened immune system

  • Foreign material, including medical devices (such as an artificial joint or heart valve), in their body

The immune system may be weakened when people take medications that suppress the immune system Some Drugs That Can Cause Immunodeficiency Some Drugs That Can Cause Immunodeficiency (immunosuppressants), such as chemotherapy or medications used to prevent rejection of an organ transplant, or when they have a disorder that causes immunodeficiency Disorders That Can Cause Immunodeficiency Disorders That Can Cause Immunodeficiency , such as AIDS. People who spend many days in an intensive care unit can develop a weakened immune system because of medical procedures, underlying disorders, undernutrition, or a combination.

Except for some superficial skin infections, fungal infections are rarely passed from one person to another.

Fungal infections can affect only one area of the body (localized) or many areas of the body (systemic).

Localized fungal infections sometimes occur when the mix of other microorganisms (eg, bacteria) that normally live in certain parts of the body (also called the microbiome) is out of balance. For example, certain types of fungi (such as Candida) are normally present on body surfaces or in the intestine. The bacteria normally present in the digestive tract and vagina limit the growth of these fungi in those areas. When people take antibiotics, the helpful bacteria can be killed, allowing the fungi to grow unchecked. The resulting overgrowth of fungi can cause symptoms, which are usually mild. As the bacteria grow back, the balance is restored, and the problem usually resolves.

Systemic fungal infections affect organs such as the lungs, eyes, liver, and brain and also can affect the skin. They typically occur in people who have a weakened immune system.

Fungal infections are either

Opportunistic fungal infections

Opportunistic fungal infections take advantage of a weakened immune system. Thus, they usually occur in people whose immune system is weakened by disorders such as AIDS or by medications that suppress the immune system. Opportunistic fungal infections occur worldwide.

Examples of opportunistic fungal infections include

Opportunistic fungal infections can be very aggressive, spreading quickly to other organs and are often fatal.

Risk Factors for Developing Opportunistic Fungal Infections

Use of medications that suppress the immune system

  • Cancer chemotherapy

  • Corticosteroids

  • Medications to prevent rejection of an organ transplant, such as azathioprine, methotrexate, and cyclosporine

  • Tumor necrosis factor inhibitors (used to treat rheumatoid arthritis and related disorders)


Primary fungal infections

Primary fungal infections can occur in people with a normal immune system, sometimes with serious consequences. These infections usually occur after people inhale fungal spores, which can cause pneumonia to develop in the lungs as the first sign of infection.

Certain primary fungal infections are more common in certain geographic areas, as in the following examples:

The time from transmission to development of an infection varies, so travelers may develop symptoms after returning from these areas.

Because many primary fungal infections develop slowly, months or years may pass before people seek medical attention. Typically, if the immune system is normal, fungal infections do not spread to organs deep in the body.

Diagnosis of Fungal Infections

  • Culture and examination of a sample

  • Blood tests

If doctors suspect a primary fungal infection, they ask people questions that can help with the diagnosis, such as the following:

  • Where they have traveled and lived to determine whether they may have been exposed to certain fungi, even if they were exposed years ago

  • Whether they are taking any medications that can suppress the immune system

  • Whether they have a disorder than weakens the immune system

Doctors then take a sample to be grown in a laboratory (cultured Culture of Microorganisms Infectious diseases are caused by microorganisms, such as bacteria, viruses, fungi, and parasites. Doctors suspect an infection based on the person's symptoms, physical examination results,... read more ) and examined under a microscope. The sample may be sputum or blood, but, occasionally, doctors must take a sample from the lungs. To take a sample from the lungs, doctors insert a flexible viewing tube (a bronchoscope Bronchoscopy Bronchoscopy is a direct visual examination of the voice box (larynx) and airways through a viewing tube (a bronchoscope). A bronchoscope, a thin viewing tube with a light, has a camera at the... read more Bronchoscopy ) through the mouth and into the airways. Fluid is squirted through the tube, then suctioned back into the tube, bringing cells and any fungi (or other microorganisms) with it. Sometimes biopsy Needle Biopsy of the Pleura or Lung A needle biopsy is a procedure in which a biopsy needle is inserted into the lung or through the membrane surrounding the lung (pleura) and is used to remove a piece of tissue for examination... read more or surgery is necessary to obtain a sample.

If the diagnosis is unclear, blood tests may be done. These tests check for antibodies Antibodies One of the body's lines of defense ( immune system) involves white blood cells (leukocytes) that travel through the bloodstream and into tissues, searching for and attacking microorganisms and... read more Antibodies (which are produced by the person's immune system in response to foreign substances, including fungi), antigens Overview of the Immune System (molecules from foreign substances that can trigger an immune response in the body), or other evidence of the fungi.

Treatment of Fungal Infections

  • Antifungal medications

Several medications effective against fungal infections are available, but the structure and chemical makeup of fungi make them difficult to kill.

Antifungal medications may be applied directly to a fungal infection of the skin or other surface, such as the vagina or inside of the mouth. Antifungal medications may also be taken by mouth or injected when needed to treat more serious infections.

For serious infections, several months of treatment are often needed.

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