In April 2011, pregnant women exhibiting severe lung disease symptoms were consecutively admitted to the emergency room and respiratory department of a university hospital in Seoul. Medical staff investigated other potential causes, as no lethal bacteria or viruses were detected in the patients. A 34-year-old female patient died on May 10, followed by three more women in June. During the same period, seven infants also died from similar symptoms. As comparable cases were reported nationwide, the Korea Disease Control and Prevention Agency (KDCA) launched an epidemiological investigation.