Federal health inspectors began inspecting the NECC plant October 1. Inspectors found foreign particles in unopened vials, and after testing one of them, they determined the substance was a fungus.

The investigation is still under way.

Nearly 10% of drugs administered in the United States come from compound pharmacies, according to a 2003 Government Accountability Office report.

Drugs manufactured by compound pharmacies do not have to go through FDA-mandated premarket approval. Instead, state health pharmacy boards are responsible for the oversight and licensing of these pharmacies.

Compound pharmacists create customized medication solutions for patients for whom manufactured pharmaceuticals won't work, according to the International Academy of Compounding Pharmacists.

Meningitis is an inflammation of the protective membranes covering the brain and spinal cord. It is usually caused by an infection, frequently with bacteria or a virus, but it can also be caused by less common pathogens like fungi, according to the CDC.

Fungal meningitis is very rare and, unlike viral and bacterial meningitis, is not contagious.

Symptoms of fungal meningitis are similar to symptoms of other forms of meningitis, but they often appear more gradually and can be very mild at first, the CDC says.

Dr. William Schaffner, chairman of the Department of Preventive Medicine at Vanderbilt University Medical Center, told CNN that fungal infections are not usually mild. He said when a fungus invades small blood vessels, it can cause them to clot or bleed, which can lead to symptoms of small strokes.

In addition to typical meningitis symptoms like headache, fever, nausea and stiffness of the neck, people with fungal meningitis may also experience confusion, dizziness and discomfort from bright lights. Patients might just have one or two of these symptoms, the CDC says.

Health officials say any patients who received an injection at one of the facilities beginning May 21 and who began showing symptoms between one and three weeks after being injected should see their doctor right away.

The earlier a patient gets treatment, the more likely he or she will survive.

Patients are treated with anti-fungal medication, which is given intravenously, so patients have to be admitted to the hospital, the CDC said. Patients may need to be treated for months.

The FDA is urging anyone who has experienced problems following an injection with the NECC product to report it to MedWatch, the FDA's voluntary reporting program, by phone at 1-800-FDA-1088 or online at www.fda.gov/medwatch/report.htm.