Of the 170 people sickened in the outbreak, all but one have a rare fungal form of meningitis after receiving suspect steroid shots for back pain, the CDC said. The other case is an ankle infection discovered in Michigan; steroid shots also can be given to treat aching knees, shoulders or other joints.

Fungus has been found in at least 50 vials of an injectable steroid medication made at a specialty compounding pharmacy in Massachusetts, investigators said. Health authorities haven’t yet said how they think the medication was contaminated, but they have ruled out other suspects — other products used in administering the shots — and the focus continues to be on that pharmacy, the New England Compounding Center.

Compounding pharmacies traditionally supply products that aren’t commercially available, unlike the steroid at issue in the outbreak. And Dr. Madeleine Biondolillo of the Massachusetts Department of Public Health said it appears the company violated state law governing those pharmacies, which aren’t supposed to do large-scale production like a drug manufacturer. Instead, they’re supposed to produce medication for patient-specific prescriptions, she said.

"This organization chose to apparently violate the licensing requirements under which they were allowed to operate," she told reporters Thursday.

Company officials weren’t immediately available to comment Thursday but earlier this week declined comment except to say they were cooperating with the investigation.

Idaho becomes the 11th state to report at least one illness. The others are Florida, Indiana, Maryland, Michigan, Minnesota, New Jersey, North Carolina, Ohio, Tennessee and Virginia.

Last month, after illnesses began coming to light, the company recalled three lots of the steroid medicine — known as preservative-free methylprednisolone acetate— that were made in May, June and August. The recall involved about 17,700 single-dose vials of the steroid sent to clinics in 23 states.

It’s not known if all or just some of the vials were contaminated, or how many doses were administered for back pain or for other reasons. Those given joint injections are not believed to be at risk for fungal meningitis, which is an inflammation of the lining of the brain and spinal cord. A back injection would put any contaminant in more direct contact with that lining.

Symptoms of meningitis include severe headache, nausea, dizziness and fever. The CDC said many of the cases have been mild, and some people had strokes. Symptoms have been appearing between one and four weeks after patients got the shots, but CDC officials on Thursday warned at least one illness occurred 42 days after a shot.

The fungus is difficult to grow in lab analyses, and health officials on Thursday issued an unusual piece of advice to doctors: If a patient who got the injection starts to develop meningitis symptoms, he or she should be treated, even if testing is negative for the fungus.

The fungus behind the outbreaks was initially identified as Aspergillus, but as more testing of patients has been completed, it’s become clear that another fungus — a kind of black mold called Exserohilum — is the primary cause. As of Wednesday, CDC’s fungal disease laboratory confirmed Exserohilum in 10 people with meningitis and Aspergillus in just one.

Exserohilum is common in dirt and grasses, but this is the first time it’s been identified as the cause of meningitis, said Weber, who is managing the CDC’s response to the outbreak.

Health officials are hurriedly trying to determine the best way to treat this kind of an illness, and have settled on two very strong anti-fungal medications. Consulting with experts, they’re making a best guess as to the dosage and length of time patients will have to be treated.

"This is new territory," Weber said.

Fungal meningitis is not contagious like the more common forms.

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Stobbe reported from New York.

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Online:

CDC information: http://www.cdc.gov/HAI/outbreaks/meningitis.html