Health officials are investigating the death of an Iowa resident on Monday as a suspected case of Lassa fever, a frightening viral illness that is rare in the United States, health officials said.

The patient returned to the United States from West Africa earlier this month. The person was not sick during the trip, so the risk to fellow passengers on the flight is “extremely low,” officials at the U.S. Centers for Disease Control and Prevention said. The virus is not transmitted through casual contact and patients are not assumed to be infectious before symptoms appear.

The patient was hospitalized in isolation at the University of Iowa Health Care Medical Center. Testing by the Nebraska Laboratory Response Network early Monday showed the patient was presumed positive for Lassa fever. If the results are confirmed, the Iowa case would be the ninth known case of Lassa fever since 1969 in travelers returning to the United States from areas where the disease occurs.

The CDC is assisting Iowa health officials in identifying people who had contact with the patient after symptoms began. Symptoms are typically mild and include fever, fatigue and headache. Some people may experience vomiting, difficulty breathing, facial swelling, and pain in the back, chest, or stomach.

Those identified as close contacts will be monitored for 21 days.

State and local health officials are working to determine how the patient, who they have not identified, became infected. Initial information suggests that the patient had contact with rodents in West Africa. The virus is transmitted by rodents and is transmitted to humans through contact with the urine or feces of infected animals. In rare cases, transmission between people can occur through direct contact with the blood or body fluids of a sick person, through mucous membranes, or through sexual contact.

In West Africa, approximately 100,000 to 300,000 cases of Lassa fever and approximately 5,000 deaths occur each year.

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