Fungal Diseases: Causes and Types

FROM THE LECTURE SERIES: An Introduction to Infectious Diseases

By Barry C. Fox, M.D., University of Wisconsin

Fungal diseases usually do not involve humans directly, but they have played a major role in human history. There are many sources from where a person can acquire a fungal disease. Some fungal infections are contagious and can prove to be fatal, whereas some are less harmful.

A drawing showing peasants affected by the Irish Potato Famine.
The Irish Potato Famine of 1845 was a result of an outbreak of a fungal disease. (Image: Everett Collection/Shutterstock)

The Irish Potato Famine

One of the lesser-known outbreaks of fungal disease was the Irish Potato Famine that began in 1845. The fungus wiped out Ireland’s potato crops and more than a million people died of starvation and illness brought on by malnutrition.

The potato was the staple of the Irish people and when the potatoes turned into a slimy, decaying pile of rot, many theories came up. One was that it was the result of static electricity from the new railroad locomotives.

However, the actual cause was an airborne fungus that had traveled from Mexico to Ireland in the holds of ships traveling to Ireland. The fungus was destroying the roots of the potato plant. This was possible due to the boost in global trade.

Besides hunger, many people died from diseases associated with malnutrition, which indirectly caused deficiencies in the immune system, making those affected more prone to infectious diseases.

In 1861, a scientist proved that a fungus caused the blight disease. The fungus continues to cause disease in potatoes in other countries. Currently, a more destructive fungal strain is threatening potato fields in Russia.

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Fungi and Molds

Fungi are eukaryotic cells containing a nucleus and many organelles; these are more complicated life forms. They require oxygen to live. Fungi are structurally divided into yeasts—the green circles on moldy pieces of bread in the refrigerator, or blocks of cheese that one might find in the back of the refrigerator.

Molds are composed of many hyphal elements. Hyphae are threadlike, branching tubules composed of fungal cells attached end to end. Most fungi are either yeast or mold. However, there are some fungi that are dimorphic—they exist in one shape in the human body and a different shape in laboratory culture media.


Depiction of a yeast cell.
Yeast is a type of fungi with a unicellular growth form. (Image: Rattiya Thongdumhyu/Shutterstock)

Yeast is a unicellular growth form of fungal growth. These cells can appear spherical or elliptical. They reproduce by budding. Spores are the reproducing bodies of molds.

Cell membranes of fungi are also more complex than those of bacteria. The outer cell contains a building block known as ergosterol, which is similar in structure to cholesterol, which in turn, is an ingredient of human cell membranes. This can be important since some antifungal medications may have side effects that are associated with the structural similarity between ergosterol and cholesterol.

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Types of Fungal Infections

Fungal infections are generally categorized by the depth of invasion into the human body. The different categories include surface-based skin infections, infections underneath the skin, body-wide infections associated with specific geographic areas, and then infections in patients with compromised immune systems.

Fungal infections of the skin surfaces, and of hair and nails are known as “dermatophytes.” These fungi live in the dead, outer layers of the skin, hair, and nails.

Image of a dermatologist examining an infected toenail.
Fungal infections may lead to thickened, discolored, and brittle nails. (Image: Alona Siniehina/Shutterstock)

Keratin and Fungal Infections

Keratin is the primary structural protein of these body components. Dermatophytes secrete an enzyme, keratinase, which partially digests layers of these body components. The disruption of keratin causes the loss of hair, scaling of the skin, and crumbling of the nails.

When the nails are involved, the term is known as “onychomycosis.” This leads to thickened, discolored, and brittle nails.

Fungal nail infections do become more prevalent as we age, and about half of adults over the age of 70 are affected.

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Superficial and Subcutaneous Fungal Infections

Image of skin infected by ringworm.
Superficial fungal infections lead to the formation of a ring with a raised border known as ringworm. (Image: Photo Win1/Shutterstock)

Superficial fungal infections are also nicknamed “tinea.” Tinea corporis forms a ring shape with a red raised border and is known as “ringworm.” When these dermatophytes involve the scalp, they are known as tinea capitis.

When they involve the foot, they are known as tinea pedis, causing “athlete’s foot.” The athlete’s foot causes cracking and peeling of skin between the toes. Subcutaneous fungal infections usually enter the body following an injury to the skin rather than a digestive enzyme. They usually remain localized to the tissues just below the top layers of the skin or spread to local lymph nodes.

Candida Albicans

One important yeast that can cause either superficial or subcutaneous fungal infection is known as “Candida albicans.” Candida species, like bacteria, are part of the human body’s normal microbiota. When antibiotics can kill the bacteria but not the yeast, candida can take over and expand their growth without competition.

Candida causes diaper rash in newborns, as well as an overgrowth in the vaginal area after antibiotics while leading to a white discharge. Sometimes a white pattern is seen in the mouth known as “oral thrush” after antibiotic therapy. Candida can also be involved in body-wide infections in patients who are hospitalized and subjected to extensive antibiotic therapy and invasive medical devices such as intravenous catheters.

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Rose Gardener’s Disease

Several years ago in Wisconsin, the health department was notified of four employees at a garden center with suspected sporotrichosis. All four were involved in making floral arrangements for gravesites.

Each had developed an ulcerating lesion on the hand or the wrist that was originally thought to be a bacterial infection. Next, they developed an ascending infection of the lymph system, and fungal cultures from the lymph nodes grew sporothrix.

While the investigators tried to find the cause, the 4 employees were cured with an oral antifungal medication known as “ketoconazole.” Samples of the sphagnum moss used during the production of the floral arrangements at the garden center were tested and were positive for sporothrix. The sphagnum moss had been harvested from bogs located in central Wisconsin and was purchased from one wholesale dealer.

Common Questions about Fungal Diseases

Q: What caused the Irish Potato Famine of the 1840s?

The cause of the Irish Potato Famine was an airborne fungus that had traveled from Mexico to Ireland in the holds of ships traveling to Ireland.

Q: What are fungi?

Fungi are eukaryotic cells containing a nucleus and many organelles that require oxygen to live.

Q: Identify the categories of fungal infections.

The different categories of fungal infections are: surface-based skin infections, infections underneath the skin, body-wide infections associated with specific geographic areas, and infections in patients with compromised immune systems.

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