(Reuters) - A blood infection known as Elizabethkingia has been linked to 18 deaths in Wisconsin and local health officials and the U.S. Centers for Disease Control and Prevention are investigating the source, officials said on Wednesday.

The infection has been found in 48 people between Nov. 1, 2015 and March 9, according to the Wisconsin Department of Health Services (DHS) website. Most of the patients, in southeastern and southern Wisconsin, are older than 65 and have a history of at least one underlying serious illness.

Health officials are unaware how the outbreak began, but were working with eight CDC investigators to determine the cause, DHS and CDC spokeswomen said.

Elizabethkingia are bacteria that are rarely reported to cause illness in humans, and can sometimes be found in the respiratory tract.

Symptoms of illness that can result from exposure to the bacteria can include fever, shortness of breath, and chills or cellulitis. Confirmation of the illness requires a laboratory test.

Officials are not sure whether Elizabethkingia contributed to or caused the death of the 18 people, even though they tested positive for the infection, according to the Wisconsin DHS.