How does a steroid become contaminated with fungus, and how does the fungus hurt you?

It's unclear in this case how the steroids became infected.

Dr. William Schaffner, chairman of the Department of Preventive Medicine at Vanderbilt University Medical Center, said that when a company is creating a sterile pharmaceutical product, the facility as well as workers must adhere meticulously to good manufacturing practices.

"There's a whole book on these practices," he told CNN Monday. "Clearly, there was some violation."

Fungal spores are in the air around us, he points out, and they don't cause harm when they are inhaled. "When they are injected, it's a different story," he said.

If injected, the fungus invades small blood vessels and can cause them to clot or bleed. That can lead to stroke-like symptoms, Schaffner said.

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, Schaffner said, and they might not present for more than a month.

"That can make it very difficult to diagnose," he said. "That could be a huge challenge in this case."

Is the practice of compounding common in the United States?

Physicians and clinics are increasingly getting material from compounding pharmacies because they typically sell at a much lower cost than major drug manufacturers, according to Outterson.

"There's a lot of compounding that's going on. And there's been an increase in reliance on these pharmacies to deliver a product that couldn't be had otherwise -- for financial or medical reasons," he said.

One to three percent of all prescriptions dispensed in the United States are compounded on prescriptions for individual patients, according to the International Academy of Compounding Pharmacists which has 2,700 local pharmacists who provide a compounding service.

The academy estimates that there are 7,500 pharmacies in the United States that specialize in complex compounding services.

"It's good because drugs are expensive and that's a reality we have to deal with," Outterson said. "But it also begs the question, 'Should the FDA be regulating compounding pharmacies?' Some would say yes."

Why doesn't the FDA regulate compounding pharmacies?

All pharmacies are regulated by the state where they are located, but regulations vary state by state, Outterson said.

The FDA has tried to oversee these pharmacies, but the attempt failed. The FDA developed rules for compounding but litigation led to a U.S. Supreme Court ruling in 2002 that struck down the idea, Outterson said.

Congress has been unable to reestablish the FDA's authority over the pharmacies since.

What are officials doing about the outbreak?

The FDA, the CDC and the Massachusetts Board of Registration in Pharmacy are all investigating the outbreak.