Health officials are investigating nearly 100 suspected cases of a potentially serious fungal infection at a Michigan paper mill, nearly a month after reports of «atypical» pneumonia were raised.

The investigation began in March when 15 Escanaba Billerud Paper Mill employees appeared to be ill earlier in the year, Public Health Delta & Menominee Counties said in a statement at the time. Later, tests indicated that the cases were not pneumonia, but a fungal infection called blastomycosis.

At least 19 cases were confirmed at the paper mill as of Friday, with another 74 probable cases, the public health department said last week.

Blastomycosis is caused by a fungus that lives primarily in moist soil and decaying organic matter, such as wood, and spreads through inhalation of spores, according to the Centers for Disease Control and Prevention. Symptoms can include fever and cough, but people with weaker immune systems may have more severe cases.

The infection has the potential to spread to the lungs and other organs, the CDC says.

Brian Peterson, the plant’s vice president of operations, thanked the federal, state and local health agencies that assisted in the investigation in a statement released Friday.

«Although the source of the infection has not been established, we continue to take this matter very seriously and are following the recommendations of government and health officials and implementing numerous proactive steps to protect the health and safety of our employees, contractors and visitors. «. Peterson said.

A spokesman for Billerud’s corporate office did not immediately respond to a request for comment on Tuesday. Billerud is a multinational paper and packaging company headquartered in Sweden.

Its Michigan plant was originally founded in 1911 as the Escanaba Pulp and Paper Company, located on the bank of the Escanaba River. The river connects to Little Bay de Noc facing Lake Michigan.

Blastomyces, the fungus that causes the infection, is typically found in areas surrounding the Great Lakes, as well as the Ohio, Mississippi and St. Lawrence rivers, according to the CDC.

More than 800 people are employed at the mill, which has the capacity to produce approximately 730,000 tons of paper per year, according to the Billerud website.