At least 18 people have died in Wisconsin from a rare bacterial outbreak.

The Wisconsin Department of Health Services confirmed at least 44 cases of Elizabethkingia anophelis were reported from Nov. 1, 2015, to March 2, 2016, with 18 deaths attributed to the outbreak, ABC News reports. Most of the infected are 65 and older, but everyone has at least one serious underlying health condition.

Elizabethkingia anophelis is a naturally occurring bacteria found in soil, fresh water and reservoirs, according to health officials. An outbreak of the bacteria is very rare.

Symptoms of infection include fever, shortness of breath, chills or redness on the skin.

The source of the outbreak is unknown and an investigation is underway by state and federal health officials.

“Determining the source of the bacteria affecting patients in Wisconsin is a complex process,” state health officer Karen McKeown said in a statement. “While we recognize there will be many questions we cannot yet answer, we feel it is important to share the limited information we have about the presence of the bacteria, as we continue our work to determine the source.”

Elizabethkingia anophelis was discovered in 2011. Investigators used advanced molecular detection to conclude it was responsible for the Wisconsin infections and are testing environmental samples, including water, to find the source of the outbreak, according to a CDC spokesman.

On March 4, Waukesha Memorial Hospital confirmed they had six patients with positive blood cultures of a Elizabethkingia anophelis infection, WITI reports. In recent weeks, all of the patients had gone to the emergency room at Waukesha Memorial Hospital with symptoms.

The infected are spread across the state, with cases being reported in Columbia, Dane, Dodge, Fond du Lac, Jefferson, Milwaukee, Ozaukee, Racine, Sauk and Washington Counties.