But he added that not everyone who got the steroid injection will develop meningitis -- an inflammation of tissue surrounding the brain and spinal cord -- but it's hard to know how many will.
Also, he and other experts said some of the symptoms associated with this rare form of meningitis are unusual.
"One of the things we are just learning about these patients is that they can present with minor stroke-like symptoms, which would include slurred speech and unsteady gait," Schaffner said.
Stroke is not usually associated with either bacterial or viral meningitis, said another expert, Dr. Pascal James Imperato, a dean at the School of Public Health of SUNY Downstate Medical Center in New York City.
Infected patients must receive intravenous drugs in a hospital setting, Imperato said.
Treatment can take weeks if not months, because these infections are difficult to treat, Schaffner explained. And the drugs can have severe side effects, including affecting kidney function, he added.
While some infected patients are doing well, others are in intensive care and may die, according to published reports.
Although the steroid is the primary target of investigation, health officials haven't ruled out the antiseptic and anesthetic used during the injections as a possible cause of the outbreak, experts said.
Dr. Marc Siegel, associate professor of medicine at NYU Langone Medical Center in New York City, said the meningitis outbreak underscores the importance of sterilization procedures in intravenous and intramuscular shots.
"I believe that this could have been prevented by more vigilance," Siegel said.
However, he added that he doesn't expect the number of infected patients to balloon in the near future.
"This is not going to be an epidemic, because the fungus is weak and there isn't a reservoir," Siegel said. "But there will continue to be isolated cases over the next several weeks because of the long incubation period."
Specialty manufacturers like New England Compounding Center make drug solutions that aren't available from the big pharmaceutical companies, but they aren't subject to the same rigorous safety standards, such as FDA oversight.
More information
The U.S. National Library of Medicine has more about injections for back pain.
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