Two employees at Frankfurt Airport died after contracting malaria in a rare outbreak, German authorities have said.

The first death occurred in early July, though malaria was confirmed as the cause only after the patient died, according to the Frankfurt health office. The second death, in August, was announced on Wednesday by airport operator Fraport.

In total, six airport employees were infected with malaria, and the four survivors are recovering after treatment, the health office said.

Germany’s Robert Koch Institute earlier confirmed four cases among airport staff who fell ill between 4 and 6 July. Officials believe one or several infected Anopheles mosquitoes arrived from a malaria-endemic region, most likely aboard an aircraft.

Fraport has set up mosquito traps at the airport and is having captured insects examined in a laboratory. Employees have been advised to seek medical care if they develop typical malaria symptoms and to inform doctors that they work at the airport.

The Frankfurt health office described so-called “airport malaria” as “a very rare” form of transmission outside malaria-endemic areas.

“Transmission from person to person plays no role in airport malaria,” a spokesperson said.

Malaria is transmitted by Anopheles mosquitoes that have themselves become infected. The World Health Organization says the disease can be life-threatening if left untreated. Symptoms include fever, chills, headaches, confusion, seizures and breathing difficulties.