Since November, 54 people in Wisconsin have one by one fallen ill with an obscure kind of bacteria called Elizabethkingia. Fifteen have died from the infection.

Elizabethkingia is common in the environment—in soil, in water—but it rarely gets people sick. Cases pop up in ones and twos, usually in people with weak immune systems, says Michael Bell, deputy director of the Centers for Disease Control and Prevention’s healthcare quality division. “The fact that we’re seeing more than four dozen cases, that is a very large outbreak.” In fact, an outbreak of this size for Elizabethkingia, named for the bacteriologist who first isolated it in the 1950s, is unprecedented. The bacteria infects the blood, causing fever, chills, and shortness of breath.

What’s going on? It’s a mystery for the CDC’s disease detectives, a corps of about 70 doctors and epidemiologists who specialize in tracing outbreaks, from foodborne illnesses to Ebola. Wisconsin’s public health department contacted the CDC for help in February, and the federal agency now have team of seven people on the ground in Wisconsin.

At first, the CDC suspected the tap water. Just this January, the CDC’s emerging diseases journal published a report about a nearly two-year long Elizabethkingia outbreak in a London critical care unit that ended up originating with contaminated taps in hospital sinks. But the tap water in Wisconsin turned up negative for the bacteria.

Adding to the mystery, this outbreak doesn't match the pattern of other infections, which appeared in clusters in the same facility. Most of Wisconsin patients were elderly; some lived in nursing homes and others had gone to the hospital, but they lived across 12 different counties. At the same time, the genetic signature of the bacteria points to a single source. The infection seems, to use the language of epidemiology, to be community-acquired. This makes tracing a source more difficult: The CDC’s officials can’t just order up medical records from a single hospital and test the area exhaustively.

So the disease detectives have fanned out across the state, knocking on doors of patients. The CDC's staff asks questions, filling in the blanks with samples of personal care products like lotions or wipes that might harbor the bacteria. The freshest cases take precedence. “With older cases,” says Bell, “you’re dealing with materials already discarded and memories already faded.”

The questions they ask are methodical and standardized. For example, if you want to ask if someone has eaten lettuce, “most people will just think of salad," says Bell. So instead, the CDC might ask if you got lettuce or a sandwich with lettuce. "When you point it out, it can jog their memory. You make sure that everyone gets a consistent shake answering the questions,” says Bell. And like good scientists, they talk to a control group of people who live in the same communities but have not gotten sick. Then they start crossing off hypotheses. A 10 hour day on the road might end with several more hours of data entry.

At this point, CDC is still investigating water—though other sources like water used on produce, which requires tracing it back to grocery stores or farms. “The amount of potential exposure sources is very large,” Bell says.

The outbreak is still going on, but Bell sees some hope in the numbers. Elizabethkingia is resistant to some but not all antibiotics, and the number of deaths has held steady as doctors have learned how to treat the once-obscure bacteria. Wisconsin may be seeing a new bacterial threat, but at least the CDC is learning how to handle it.