The Rhizopus species are usually the cause of mucormycosis. In descending order, the other genera with mucormycosis-causing species include Rhizomucor, Cunninghamella, Apophysomyces, Saksenaea, Absidia, Mucor, Syncephalastrum, Cokeromyces and Mortierella.

Most people are exposed to these fungi on a daily basis. However, individuals who have a compromised immune system are most likely to develop an infection. In most cases, the infection occurs after fungus spores are inhaled, but an infection can also develop after ingestion. There have also been reports of infection developing after wounds were exposed to non-sterile medical tape or wooden splints.

When spores are deposited in the nasal cavity, rhinocerebral disease develops. When the spores are deposited into the lungs, pulmonary disease develops. When the fungus is exposed to open sores on the skin, cutaneous disease develops. Ingestion leads to gastrointestinal disease, especially in patients who are malnourished.

Once the spores are in the body, they begin to grow, and fungal hyphae invade the blood vessels, which produces tissue infarction, necrosis (cell death) and thrombosis (blood clots). In a healthy person, neutrophils would destroy the fungus. However, in an immunocompromised person there are not enough neutrophils to destroy the fungus and prevent infection.

Most cases of mucormycosis are acute surgical emergencies. However, cases may also be chronic, with symptoms gradually developing over the course of four weeks or longer.

Rhinocerebral mucormycosis: Common symptoms of rhinocerebral mucormycosis include acute sinusitis, fever, eye swelling, protrusion of eye orbit, dark nasal eschar (scabbing), progressive cellulitis, facial pain, retinal artery thrombosis, redness of skin overlying sinuses and nasal stuffiness that progresses to black discharge.

Late symptoms such as diplopia (double vision) and visual loss indicate that the infection has spread to the orbital nerves and vessels. Patients with these symptoms usually have a poor prognosis.

Pulmonary mucormycosis: The symptoms of pulmonary mucormycosis are nonspecific. Symptoms often include fever, cough, rales (bubbling or rattling sounds when air moves through fluid-filled airways) and shortness of breath. Hemoptysis (blood in the sputum) may occur if necrosis is present.

Gastrointestinal mucormycosis: Some patients with gastrointestinal mucormycosis experience tenderness to palpation, abdominal pain, distension (bloating), nausea or vomiting blood. Hematochezia (blood in stools) may also occur.

Central nervous system: If the infection affects the central nervous system, symptoms may include decreased consciousness and focal neurologic signs, such as cranial nerve deficits.

Cutaneous mucormycosis: Cutaneous mucormycosis causes a single, painful, hardened area of skin that may have a blackened central area.

General: Mucormycosis should be suspected if symptoms appear in patients with compromised immune systems. A tissue biopsy is the standard diagnostic test for all types of mucormycosis.

Tissue biopsy: A tissue biopsy from the affected area of the body is the best way to definitively diagnose the condition. During the procedure a needle is inserted into the patient and a small piece of tissue is removed. The tissue sample is then analyzed to determine whether the infection is present.

Imaging studies: Depending on the site of involvement, Computerized Tomography (CT) scans or Magnetic Resonance Imaging (MRI) may be performed to detect infected tissue inside the body. These tests help healthcare professionals assess the tissue damage and determine whether surgery is necessary.

Lumbar puncture (spinal tap): A lumbar puncture (spinal tap) may be performed if it is suspected that the central nervous system is involved. During the procedure, the patient first receives a local anesthetic. Then a needle is inserted into the lower back and a sample of cerebrospinal fluid (CSF) is removed. Patients who have central nervous system mucormycosis may have elevated protein levels and modest mononuclear pleocytosis in the CSF. A CT scan should precede a lumbar puncture to ensure that this procedure is safe.

Loss of neurological function can occurs when the infection affects the central nervous system.

Mucormycosis can cause blood clots in the brain or lung vessels (thrombosis).

If the infection spread to the optic nerve, it may result in permanent blindness.

Individuals who are immunocompromised are at the greatest risk of developing mucormycosis. Historically, patients with uncontrolled diabetes mellitus (especially with ketoacidosis) are at the greatest risk for infection. This is because funguses thrive in acidic environments. Immunocompromised patients, especially those who have cancer (particularly those who are neutropenic, have received broad-spectrum antibiotics or are undergoing chemotherapy or radiation therapy), liver problems, chronic renal failure, are taking immunosuppressive agents, are malnourished, have burns and recently suffered from trauma, are at an increased risk.