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Overview of Fungal Infections


Sanjay G. Revankar

, MD, Wayne State University School of Medicine

Last full review/revision Sep 2019| Content last modified Sep 2019
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Topic Resources

Fungi are neither plants nor animals. They were once thought to be plants but are now classified as their own kingdom. Some fungi cause infections in people.

  • Because fungal spores are often present in the air or in the soil, fungal infections usually begin in the lungs or on the skin.

  • Fungal infections are rarely serious unless the immune system is weakened, usually by drugs or medical disorders.

  • Fungal infections usually progress slowly.

  • Antifungal drugs may be applied directly to the infected site or, if the infection is serious, taken by mouth or injected.

Fungi can grow in two forms:

  • Yeasts: Single round cells

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

Some fungi go through both forms during their life cycle.

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 their own kingdom—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 types 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 drugs that suppress the immune system (immunosuppressants), such as chemotherapy drugs or drugs used to prevent rejection of an organ transplant, or when they have a disorder that causes immunodeficiency, such as AIDS.

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


Risk Factors for Developing Fungal Infections

Use of drugs that suppress the immune system

  • Cancer chemotherapy drugs

  • Corticosteroids

  • Drugs 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)


Types of fungal infections

Fungal infections are often described as

  • Opportunistic

  • Primary

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

Opportunistic fungal infections take advantage of a weakened immune system. Thus, they usually occur in people with a weakened immune system, such as those with AIDS or those taking drugs that suppress the immune system. Opportunistic fungal infections occur worldwide. Typical opportunistic fungal infections include

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

Primary fungal infections can occur in people with a normal immune system, sometimes with serious consequences.

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

  • Histoplasmosis is especially common in the Ohio and Mississippi River valleys, central New York, and Texas in the United States. It also occurs in parts of Central and South America, Africa, Asia, and Australia.

  • Blastomycosis is particularly common in the eastern and central United States. It also occurs in Africa and in the Saint Lawrence River valley of Canada.

  • Coccidioidomycosis occurs almost exclusively in the Southwest and the central valley of California in the United States and in northern Mexico and Central and South America.

  • Paracoccidioidomycosis occurs mainly in certain parts South and Central America.

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.

Localized fungal infections affect only one area of the body. They sometimes occur when the normal balances that keep fungi in check are upset. 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.

Localized fungal infections typically involve the skin and nails, vagina, mouth, or sinuses.


  • 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, perhaps years previously

  • Whether they are taking any drugs 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) 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) 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 or surgery is necessary to obtain a sample.

If the diagnosis is unclear, blood tests may be done. These tests check for antibodies (which are produced by the person's immune system in response to foreign substances, including fungi), antigens (molecules from foreign substances that can trigger an immune response in the body), or other evidence of the fungi.

Tests that detect genetic material in microorganisms are also done for some fungal infections.


  • Antifungal drugs

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

Antifungal drugs may be applied directly to a fungal infection of the skin or other surface, such as the vagina or inside of the mouth. Antifungal drugs 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.


Drugs for Serious Fungal Infections


Common Uses

Some Side Effects

Amphotericin B

Most fungal infections

Chills, fever, headache, vomiting, a low potassium level* in the blood, kidney failure, and anemia




Nausea, diarrhea, headache, liver inflammation, inflammation of the vein, and rash


Candidal and other fungal infections, including coccidioidomycosis and cryptococcal infections

Nausea, rash, and liver inflammation


Candidal and cryptococcal infections

Nausea, vomiting, and bone marrow damage


Nausea, vomiting, and liver inflammation


Nausea, diarrhea, liver inflammation, rash, headache, dizziness, a low potassium level* in the blood, high blood pressure, accumulation of fluid (edema), and heart failure


Nausea, diarrhea, liver inflammation, rash, headache, dizziness, a low potassium level* in the blood, high blood pressure, accumulation of fluid (edema), and heart failure


Aspergillus, candidal, and many other fungal infections

Nausea, vomiting, rash, and liver inflammation


Aspergillus and candidal infections, fusariosis, and scedosporiosis

Temporary disturbances in vision (such as blurred vision, changes in color vision, and sensitivity to light), nausea, vomiting, rash, and liver inflammation

* A low potassium level (hypokalemia) can cause muscle weakness, cramping, and twitches and abnormal heart rhythms.

Flucytosine is usually used with other drugs.

Drugs Mentioned In This Article

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