A mysterious cluster of deadly bloodstream infections in Wisconsin has set state and federal disease detectives scrambling to understand why a common and generally harmless microbe is suddenly wreaking such havoc.

The outbreak has killed 15 people and sickened 33 others in a cluster of counties surrounding Milwaukee.

Public health authorities know what’s causing the illnesses: a species of bacteria named Elizabethkingia anophelis.

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But they don’t know how it’s infecting so many people, or why it’s proving so deadly. The microbe can resist common first-line antibiotic treatments, though it’s not a true “superbug” as it is susceptible to other medicines.

While most of the patients in Wisconsin are elderly, and all have a history of serious illness, many have not had any recent contact with hospitals, suggesting they are becoming infected in the community, which is unheard of for this type of bacteria. The outbreak started last November and the case count continues to grow, with four new cases identified last week. Symptoms include fever, chills, and shortness of breath.

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State and federal investigators are testing tap water, skin care products, and other products patients may have ingested or come in contact with as a possible source of contamination. The vast majority of specimens share a matching genetic fingerprint, indicating a common source for the outbreak.

“The fact that we don’t know what it is yet is tremendously frustrating,” said Dr. Michael Bell, deputy director of the division of healthcare quality promotion at the Centers for Disease Control and Prevention.

Most states see fewer than a dozen cases of Elizabethkingia infection each year, Bell said. It is highly unusual for nearly 50 cases to be identified in such a small geographic area.

The outbreak poses a particular challenge to investigators because scientists know little about Elizabethkingia, which is named for the CDC scientist who discovered the species.

The organisms are known to be ubiquitous in soil and in the guts of Anopheles gambiae mosquitoes, and they thrive in moist biofilms — clumps of bacteria and sticky substances that adhere to environmental surfaces and medical devices. Even strong disinfectants, like hydrogen peroxide, don’t wipe them out.

But Elizabethkingia species have only recently been identified as a cause of human disease, with the first published description of an outbreak reported in 2010.

Since then, a handful of other clusters have been described, all in hospital intensive care units and neonatal wards. In a recent outbreak in a British hospital, for instance, the infections were traced to contaminated taps in sinks near patient beds.

The cluster of cases in Wisconsin is the first reported outbreak of this particular species of Elizabethkingia bacteria. State health officials report that most victims are older than 65.

Milwaukee was the site of another cluster of mysterious fatalities back in 1993, when 69 residents died and tens of thousands fell ill with gastrointestinal symptoms. The outbreak was ultimately traced to an intestinal parasite, Cryptosporidium, that contaminated the city’s water supply.

Milwaukee has since made improvements to its water processing plants, including changes to filtration systems and disinfection procedures. State and federal officials say it’s unlikely that the current outbreak comes from contaminated tap or groundwater, but neither has been ruled out.