A mystery disease is killing dozens of children across Cambodia, and it has left both local and international health experts baffled as to its cause.

The World Health Organization (WHO) announced on Friday that Cambodia's Ministry of Health is actively investigating the outbreak.

Cambodia's Ministry of Health was alerted about the disease by Kantha Bopha Children's Hospital in the capital Phnom Penh, where the majority of the cases were hospitalized. Most of the patients were from the southern and central parts of Cambodia.

WHO reported that investigators found a total of 74 cases that have led to hospitalization between April and July 5. Some 57 of these cases had common symptoms that include fever, respiratory and neurological signs, the organization said.

There have been at least 56 deaths and health officials said a majority of the cases identified to date were in those under three years old. Many of the children died within 24 hour of being admitted, according to WHO.

Officials haven't found any indication that this illness is being spread from person to person. However, they said that tested samples were negative for H5N1 (aka the bird flu) and other influenza viruses, SARS, and Nipah.

Parents have been advised to take their children to hospital if they identify any signs of unusual illness, WHO said in a statement on its website. The government is also reinforcing awareness of good hygiene practices to the public, which includes frequent washing of hands.