A mysterious bacterial outbreak has killed 17 people and sickened at least 54 in Wisconsin, US since November 2015, officials confirmed on Thursday, March 17, 2016. Experts say this outbreak is unprecedented.

The cause of the sickness is bacteria called Elizabethkingia Anopheles. It is commonly found in the environment, but it rarely infects humans and is not known for making people sick on this scale. If the bacterium makes its way to a person’s bloodstream, it can cause fever, shortness of breath, chills, bacterial infection of the skin, and sepsis, which can be deadly.

Michael Bell, deputy director of the Centers for Disease Control and Prevention’s Division of Healthcare Quality Promotion 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."

"Wisconsin cases are the largest outbreak of Elizabethkingia recorded in published literature," Bell said.

Infections are centered in the heavily populated southeastern quarter of the state, including the Milwaukee area and surrounding suburban counties. Although previous outbreaks of Elizabethkingia in hospitals have been linked to contaminated sinks, water samples in Wisconsin have turned up negative.

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A variety of potential sources have been tested, including health care products, water sources and food, but none of these have been found to be a source of the bacteria.

The majority of patients infected are 65 or older with a history of at least one underlying serious illness, according to the state health department. Those who died all tested positive for the infection, but it’s not known if Elizabethkingia caused or contributed to their deaths, according to Washington Post.

Although isolates of Elizabethkingia have been known to be resistant to multiple antibiotics, the strains causing the current outbreak appear to still be susceptible to a number of drugs, making treatments easier.

At this time, it is unknown why and how this outbreak is spreading. Patients are spread across 12 different counties, with a range of living conditions.

Genetic analysis of the bacteria involved suggests that all of the infections are coming from a single source, and somehow infecting people all over the southeastern and southern parts of the state.

Featured image credit: ABC News (YT video screenshot)