(CNN) -- Patients whose fungal tests are negative may still be at risk in a meningitis outbreak, as the fungus involved can be difficult to detect, federal health officials said Thursday.
Cases of the non-contagious fungal meningitis rose to 170 in 11 states as of Thursday, the Centers for Disease Control and Prevention said. Fourteen people have died.
The cases have been linked to injections of contaminated steroid injection made by the Massachusetts-based New England Compounding Center.
Some 14,000 people may have received the injections, the CDC said Thursday, an increase from its previous estimate of 13,000.
Health officials have been able to contact more than 90% of the patients to warn them, officials said.
But "we aren't out of the woods yet," said Dr. J. Todd Weber, incident manager of the multistate outbreak for the CDC.
While the investigation is ongoing, 50 vials of the injection have tested positive for fungus. Those vials were found in the NECC facility, as well as in clinics that received the medication.
CDC's fungal disease laboratory has found the fungus Exserohilum in 10 meningitis patients, and the fungus Aspergillus in one patient.
Exserohilum is new territory for health officials, as it's not typically seen, Weber told reporters Thursday.
However, it can be difficult to find, meaning a patient whose fungal test is negative is not in the clear. Doctors are encouraged to be diligent in following up with patients, he said.
In addition, symptoms from Exserohilum can take a long time to develop. Patients who received the injections should remain vigilant for "at least several months after injection," Weber said.
The average period in which infections are seen is about two weeks, with the longest being 42 days, officials said. However, the longest period a patient may go before developing the disease is not known.
Curing the infection requires a long course of treatment. The CDC is recommending two anti-fungal drugs, which are very strong and can be difficult for patients to tolerate, Weber said.