Fears are mounting over a mysterious virus that left seven pregnant women hospitalized with pneumonia and caused the death of one of them on Tuesday. Health officials say they have been unable to identify the pathogen that is causing the illness and are trying their best to trace the cause.

The Korea Centers for Disease Control and Prevention has mobilized epidemiologists and other experts to find the cause of the disease, saying the simultaneous onset of acute pneumonia in several pregnant women is unprecedented.

One 35-year-old woman died after being hospitalized last month with cold symptoms that developed at the end of March. Koh Yoon-seok, a respiratologist at the University of Ulsan College of Medicine, said, "We see about two to four cases a year of pregnant women with serious lung damage or death from such ailments, but it is rare to see a collective onset of the infection."

Health officials are investigating whether pregnant women in 40 university hospitals are showing similar symptoms and are collecting samples from patients. It will take two weeks to get results from tissue samples and more than eight weeks to complete DNA analyses. "In other countries, the causes of 30 percent of pneumonia cases remains unidentified, but pregnant women are not more vulnerable and there are no signs of an epidemic," said Yang Byung-kook at the KCDC. "So there is no need for expecting mothers to panic."