NASHVILLE, Tenn. -- An outbreak of a rare and deadly form of meningitis has sickened at least 26 people in five states who received steroid injections mostly for back pain, health officials said Wednesday.

Four people have died, and more cases are expected.

Eighteen of the cases of fungal meningitis are in Tennessee where a Nashville clinic received the largest shipment of the steroid suspected in the outbreak.

The drug was made by a specialty pharmacy in Massachusetts that issued a recall last week. Investigators, though, say they still are trying to confirm the source of the infections.

Three cases have been reported in Virginia, two in Maryland, two in Florida and one in North Carolina. Two of the deaths were in Tennessee; Virginia and Maryland had one each, according to the Centers for Disease Control and Prevention.

At three clinics in Tennessee, officials are contacting the more than 900 people who received the steroid in the past three months

More new cases are almost certain to appear, said Tennessee Department of Health Commissioner John Dreyzehner. He called the situation a "rapidly evolving outbreak."

But federal health officials were not clear about whether new infections are occurring. They are looking for -- and increasingly finding -- illnesses that occurred in the last two or three months.

Meningitis involves swelling of the brain. Officials say this type of fungal meningitis is caused by a common fungus often found in leaf mold. It doesn't normally cause disease in healthy people. Fungal meningitis is not contagious like the more common viral and bacterial meningitis.

Symptoms include worsening and severe headache, nausea, dizziness and fever. Some of the patients in Tennessee also experienced slurred speech, and difficulty walking and urinating, according to Tennessee health officials.

"Some are doing well and improving. Some are very ill -- very, very seriously ill and may die," Dr. David Reagan, a Tennessee health official said of the state's patients.

The incubation period is estimated anytime between two to 28 days, so some people may not yet have fallen ill, the Tennessee health officials said.

Investigators are looking into the antiseptic and anesthetic used during the injections. Neither has been ruled out.

However, the primary suspicion is pointed toward the steroid medication. Steroid shots are common for back pain. Often they are given along with an anesthetic.

The Food and Drug Administration identified the maker of the steroid as New England Compounding Center, a specialty pharmacy in Framingham, Mass.

Last week, the company issued a recall of three lots of the steroid -- methylprednisolone acetate.

In a statement, the company said it had voluntarily suspended operations and was working with regulators to identify the source of the infection.

The outbreak was discovered about two weeks ago when Vanderbilt University's Dr. April Pettit was treating a patient who was not doing well for reasons doctors did not understand.

When the laboratory found the fungus in the patient's spinal fluid, Pettit began asking questions and learned the patient recently had steroid injections in his spine, according to Dr. William Schaffner, who chairs Vanderbilt's Department of Preventive Medicine.

"When it became clear that the infection control practices at the clinic were up to par, the steroid medication became implicated," Schaffner said.

Federal officials did not release condition reports or details on all the patients in the five states. Fungal meningitis is treated with high-dose antifungal medications, usually given intravenously in a hospital.

Seventeen of the Tennessee cases were treated at the Saint Thomas Outpatient Neurosurgery Center in Nashville. It had 2,000 vials of the suspect lots, the largest number.

That clinic voluntarily closed last month to deal with the investigation.