Clinics and medical centers rushed to contact patients who might have received apparently fungus-contaminated shots. And the Food and Drug Administration urged doctors not to use any products 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 have died, according to the Centers for Disease Control and Prevention. All had received steroid shots for back pain, a common treatment.
In one indication the outbreak could get larger, Massachusetts health officials said the pharmacy involved, the New England Compounding Center of Framingham, Mass., has recalled three lots consisting 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 -- and 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 or more -- also were treated in this manner.
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 at risk 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 underway to determine if it is the same fungus blamed in the outbreak.
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.
By far, Tennessee has 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.