Health officials are investigating Monday’s death of an Iowa resident as suspicious case of Lassa fevera frightening viral disease rarely seen in the U.S., health officials said.

The patient returned to the US from West Africa earlier this month. The person did not become ill during the trip, so the risk to airline passengers is “extremely low,” US Centers for Disease Control and Prevention officials said. The virus is not spread through casual contact, and patients are not thought to be infectious before symptoms develop.

The patient had been hospitalized in isolation at the University of Iowa Health Care Medical Center. Tests by the Nebraska Laboratory Response Network Monday morning showed the patient presumptively tested 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 US from areas where the disease is found.

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

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

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

About 100,000 to 300,000 cases of Lassa fever and about 5,000 deaths occur in West Africa each year.

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