Some 65 years ago, American microbiologist Elizabeth King peered into a microscope and discovered the “glistening, gray-white” organism that would eventually bear her name.

In the ensuing decades, the bacterium — renamed Elizabethkingia in 2005 — would fall into relative obscurity, causing only sporadic cases and the rare hospital outbreak. But late last year, something strange started happening in the state of Wisconsin.

“From the end of December to the beginning of January, we got reports from hospitals of six cases of Elizabethkingia infections,” said Karen McKeown, Wisconsin’s state health officer. “We knew right away that it was unusual.”

That cluster was the beginning of what is now considered the United States’ largest-known outbreak of Elizabethkingia, a bacterium rarely seen in humans — and four months into their investigation, disease detectives still have no idea what could be linking these cases.

Wisconsin typically sees between two and four Elizabethkingia infections in a year but since Nov. 1, health officials have identified 59 cases, with another six suspected or under investigation. Single cases have also been confirmed in nearby Michigan and Illinois and at least 20 deaths have been linked to the outbreak.

At blame is a species called Elizabethkingia anophelis, which is particularly rare in human infections — in Canada, the public health agency is aware of just one case, reported three years ago in Quebec.