The man flew back to Minneapolis-St. Paul on March 31 and was admitted to a hospital with fever and confusion the same day, according to a department statement.
A blood sample sent to the Centers for Disease Control and Prevention proved he has Lassa fever, which is rarely seen in the United States. The last case was in 2010 and was travel-related.
The man is recovering and is in stable condition, according to the Minnesota Department of Health.
Lassa fever is not transmitted through casual contact. The general public isn't at risk.
The Lassa virus is carried by rodents in West Africa and is transmitted to humans through contact with urine or droppings of infected rodents. Though rare, it can also be transmitted from person to person through direct contact with a sick person's blood or bodily fluids.
In West Africa, there are about 100,000 to 300,000 cases of Lassa fever and about 5,000 deaths annually. Eighty percent of human infections don't show any symptoms. Overall, death is rare, with only 1 percent of all cases resulting in death in areas of West Africa where infection is common.
The Minnesota Department of Health is working with the hospital, CDC and others in the investigation. Health officials are also working to notify any health care providers or staff who may have had contact with the patient.