These cases largely involve Wisconsin residents older than 65, and all were found in a dozen counties in the southeastern and southern parts of the state, according to the health department. State health officials also say that all of the people who have been infected have a history of at least one serious underlying illness.

The number of cases is remarkable, given how infrequently these infections are seen, said Michael Bell, deputy director of the Centers for Disease Control and Prevention’s Division of Healthcare Quality Promotion.

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He said the agency sees a handful of Elizabethkingia infections around the country each year, but the outbreaks rarely involve more than a couple of cases at a time. To have dozens of cases at once — and more than a third of them possibly fatal — is startling.

“It’s one of the largest [Elizabethkingia outbreaks] that I’m aware of, and certainly the largest one we’ve investigated,” Bell said.

The state health department said last week that 18 people died after testing positive for the bacteria, but officials added that it was not clear yet if the infection, other health issues or a combination that caused these deaths. Perhaps the most troubling aspect of the ongoing outbreak is that the bacteria involved all share an identical genetic fingerprint, suggesting that these infections are coming from a single source.

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“If we can find the source, the we can keep it from infecting more people,” Bell said.

An investigation involving the state’s Division of Public Health and the CDC has not yet provided answers. Wisconsin health officials say they were first told about half a dozen possible cases between Dec. 29 and Jan. 4, and began statewide surveillance a day later. A lab test is needed to confirm that someone is infected with the bacteria, which can cause fever, shortness of breath and chills.

“Determining the source of the bacteria affecting patients in Wisconsin is a complex process,” Karen McKeown, the Wisconsin state health officer, said in a statement.

But figuring that out has not been easy. The victims involved are mostly senior citizens, and all have underlying illnesses and potentially compromised immune systems. But “that’s not a surprise,” Bell said, given that’s the sort of patient more susceptible to an Elizabethkingia infection.

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Beyond that, these patients are scattered among a dozen counties in Wisconsin. Their living situations varied — a couple were in nursing homes and others were living in their own homes. While a couple of them were infected while in a hospital, others had not received any health care before their infections. No obvious pattern of medical procedures or common medical devices has emerged. Tests of the water supply have shown no signs of contamination.

“That leaves us looking at a huge number of potential risk factors,” including medications, foods and environmental sources, Bell said. “It’s frustrating. The fact that all these cases share a fingerprint has us wanting to really track down the source.”

He said the CDC currently has eight people on the ground in Wisconsin, helping local officials conduct interviews with patients and their families and scouring medical records. At the CDC’s headquarters in Atlanta, other staff members are helping conduct sophisticated lab tests that can compare the genetic makeup of specific strains of the pathogen.

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Bell said Elizabethkingia is, like other bacteria, prevalent in sources such as soil or water, though it rarely causes serious infections. Patients who are infected typically are treated with antibiotics, though Bell said the pathogen has proven to be resistant to some drugs.

So far, cases have been identified in the following counties: Columbia, Dane, Dodge, Fond du Lac, Jefferson, Milwaukee, Ozaukee, Racine, Sauk, Sheboygan, Washington and Waukesha.

The Wisconsin Department of Health Services had said that 18 people infected with the bacteria died, but on Thursday a spokeswoman said the number was lowered to 15 people. This change came as investigators continued to go through their findings, which meant that a death believed to be related to the infection could not be confirmed or that authorities found incorrect or duplicative reports, said Jennifer Miller, the spokeswoman.

This post has been updated.