Phnom Penh, Cambodia (CNN) -- Before dawn, the line of parents trying to comfort their sick children stretched around the Kantha Bopha Children's Hospital in Cambodia's capital, Phnom Penh.

Dozens of children waited their turn for treatment. It is the normal daily routine here, especially during the rainy season when mosquito-borne illnesses are most prevalent.

But over the past four months doctors inside the busy hospital have been faced with something that is not routine at all; a mysterious syndrome killing children so fast nearly all of the children infected with it die within a day or two of being admitted to the hospital.

Other hospitals in the country also began reporting similar cases -- though far fewer than the children's hospital in the capital, which is the most popular. Since April, doctors at the Kantha Bopha hospital have reported 66 cases of the illness. Of those cases only two children survived, while 64 died.