Fungi are everywhere in the environment. They are important in ecosystems, where they perform the valuable function of decomposing dead organic matter. Although many plant diseases are caused by fungi, with some exceptions, serious fungal infection of humans is relatively rare. Only a few species are adapted to survival in a human host.
Fungi and the Immune System
Although we are exposed to potentially dangerous fungi nearly every day, they are usually held in check by the immune system. Some fungi are also a part of normal microbial flora that exist in and on humans. Mild infections of the skin are common and rarely life threatening. More serious fungal infections (mycoses) can occur in people when normal flora get out of control, or fungi find a way into the body either via the lungs or through a break in the skin. The chance of a serious infection is greatly increased when normal immune functioning is impaired.
Normal Fungal Flora
The fungus Candida albicans is a normal inhabitant of the human vaginal tract. It can grow out of control under some circumstances. The fungus shares its environment with bacteria, and there should be a healthy balance between the two. If the "ecosystem" is perturbed in such a way as to favor yeast growth, infection can result. This may happen when the pH gets too low (increased acidity), or when antibiotics reduce the population of beneficial bacteria. Candida yeast infections can also result from improper hygiene, internal injury, sexual transmission or when the immune system is weakened by drugs, HIV or other disease. Candida infection of the oral cavity or skin is common in infants and immunodeficient adults. Mallassezia is another yeast normally found growing harmlessly on skin. It can cause infections that appear as an itchy rash.
Dermatophytes are fungi that have special adaptations allowing them to colonize the skin, hair and nails of humans (and other animals). These fungi do well in the warm, moist environment often found in close proximity to the body, and possess enzymes capable of digesting specific proteins found in skin, hair and nails. Superficial (not deep and penetrating) infections by dermatophytes are called tineas and are named by the part of the body affected. • Tinea corporis (general skin infection, also called ringworm) • Tinea versicolor (upper body skin) • Tinea cruris (skin of the groin and inner thighs) • Tinea capitis (scalp and head hair) • Tinea pedis (feet, known as athlete's foot)) • Tinea unguium (nails) • Tinea barbae (beard) Unless the immune system is compromised, these fungal infections are not life-threatening.
Lung infections result from breathing in fungal spores from the air. Spores are breathed in every day without harm, but if there is an underlying disease or weak immune response, the spores can germinate, allowing the fungus to colonize the lungs. The infection may stay localized in the lung and cause subclinical symptoms, or even no symptoms at all. In the normal host, fungal lung infections are often self-clearing. In the case of immune deficiency, the infection can spread from the lungs to other parts of the body to become disseminated disease. These deep mycoses can be difficult to cure and often require a long period of treatment. Transplant patients are at particular risk of infection by aspergillus beginning in the lungs and later disseminating. This susceptibility is a consequence of the immunosuppresive drugs administered prior to and during surgery and recovery.
Fungal cells can be implanted into the body when fungi are present on an object that punctures the skin. This can happen while working or recreating outdoors, or during any event in which the skin is broken or lacerated and comes in contact with organic material (such as may happen in a motorcycle accident). Infection may remain localized to the area of infection and respond to treatment, or the infection may spread. As is true in any case of disseminated fungal disease, if internal organs, bones or the brain become infected, the prognosis is not good.