Health officials are investigating the death Monday of an Iowa resident as a suspected case of Lassa fever, a frightening viral disease rarely seen in the U.S., health officials said.

The patient returned to the U.S. from West Africa early this month. The person was not sick while traveling, so the risk to fellow airline passengers is “extremely low,” officials with the U.S. Centers for Disease Control and Prevention said. The virus is not spread by casual contact and patients are not believed to be infectious before symptoms occur.

The patient had been hospitalized in isolation at the University of Iowa Health Care Medical Center. Testing by the Nebraska Laboratory Response Network early Monday showed that the patient was presumptively 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 U.S. from areas where the disease is found.

The CDC is assisting Iowa health officials to identify people who had contact with the patient after symptoms began. Symptoms are typically mild and include fever, fatigue and headache. Some people may develop 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 learn how the patient, who they did not identify, 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 urine or feces droppings of infected animals. In rare cases, it can be transmitted among people through direct contact with a sick person's blood or bodily fluids, 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.