NEW YORK (AP) — The potential scope of the meningitis outbreak that has killed at least five people widened dramatically Thursday as health officials warned that hundreds, perhaps thousands, of patients who got steroid back injections in 23 states could be at risk.

Clinics and medical centers rushed to contact patients who may have received the apparently fungus-contaminated shots. And the Food and Drug Administration urged doctors not to use any products at all from the Massachusetts pharmacy that supplied the suspect steroid solution.

It is not clear how many patients received tainted injections, or even whether everyone who got one will get sick.

So far, 35 people in six states — Tennessee, Virginia, Maryland, Florida, North Carolina and Indiana — have contracted fungal meningitis, and five of them have died, according to the Centers for Disease Control and Prevention. All had received steroid shots for back pain, a highly common treatment.

In an alarming indication the outbreak could get a lot bigger, Massachusetts health officials said the pharmacy involved, the New England Compounding Center of Framingham, Mass., has recalled three lots consisting of a total of 17,676 single-dose vials of the steroid, preservative-free methylprednisolone acetate.

An unknown number of those vials reached 75 clinics and other facilities in 23 states between July and September, federal health officials said. Several hundred of the vials, maybe more, have been returned unused, one Massachusetts official said.

But many other vials were used. At one clinic in Evansville, Ind., more than 500 patients got shots from the suspect lots, officials said. At two clinics in Tennessee, more than 900 patients — perhaps many more — did.

The investigation began about two weeks ago after a case was diagnosed in Tennessee. The time from infection to onset of symptoms is anywhere from a few days to a month, so the number of people stricken could rise.

Investigators this week found contamination in a sealed vial of the steroid at the New England company, according to FDA officials. Tests are under way to determine if it is the same fungus blamed in the outbreak.

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The company has shut down operations and said it is working with regulators to identify the source of the infection.

"Out of an abundance of caution, we advise all health care practitioners not to use any product" from the company, said Ilisa Bernstein, director of compliance for the FDA's Center for Drug Evaluation and Research.

Tennessee has by far the most cases with 25, including three deaths. Deaths have also been reported in Virginia and Maryland.

Meningitis is an inflammation of the lining of the brain and spinal cord. Symptoms include severe headache, nausea, dizziness and fever.

The type of fungal meningitis involved is not contagious like the more common forms. It is caused by a fungus often found in leaf mold and is treated with high-dose antifungal medications, usually given intravenously in a hospital.

Robert Cherry, 71, a patient who received a steroid shot at a clinic in Berlin, Md., about a month ago, went back Thursday morning after hearing it had received some of the tainted medicine.

"So far, I haven't had any symptoms ... but I just wanted to double check with them," Cherry said. "They told me to check my temperature and if I have any symptoms, I should report straight to the emergency room, and that's what I'll do."

The New England company is what is known as a compounding pharmacy. These pharmacies custom-mix solutions, creams and other medications in doses or in forms that generally aren't commercially available.

Other compounding pharmacies have been blamed in recent years for serious and sometimes deadly outbreaks caused by contaminated medicines.

Two people were blinded in Washington, D.C., in 2005. Three died in Virginia in 2006 and three more in Oregon the following year. Twenty-one polo horses died in Florida in 2009. Earlier this year, 33 people in seven states developed fungal eye infections.

Compounding pharmacies are not regulated as closely as drug manufacturers, and their products are not subject to FDA approval.

A national shortage of many drugs has forced doctors to seek custom-made alternatives from compounding pharmacies.

The New England company at the center of the outbreak makes dozens of other medical products, state officials said. But neither the company nor health officials would identify them.

The company said in a statement Thursday that despite the FDA warning, "there is no indication of any potential issues with other products." It called the deaths and illnesses tragic and added: "The thoughts and prayers of everyone employed by NECC are with those who have been affected."

A 2011 state inspection of the Framingham facility gave the business a clean bill of health.

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Associated Press writers Travis Loller in Nashville, Jay Lindsay in Boston, Randall Chase in Wilmington, Del., and AP chief medical writer Marilynn Marchione in Milwaukee contributed to this story.