Health workers are trying to determine whether a mixture of known diseases or something new is responsible for killing more than 60 children in Cambodia over the past three months, a World Health Organization expert said.

The mystery disease has killed 61 of 62 children hospitalized since April, but there's no indication it is spreading from person to person. Patients suffer a high fever, followed by severe respiratory problems that progress quickly. Some also experience neurological symptoms.

"At this stage, we cannot rule out if this is a mixture of a number of known diseases (virological, bacterial or toxicological) which have been reported as one syndrome or something new," Dr. Nima Asgari of the WHO in Phnom Penh said in an email Thursday.

The patients reported were under 7 years old and spread across several provinces in southern and central Cambodia. However, work is being done to determine whether other age groups may also be affected, he said.

Health Ministry officials alerted WHO on July 1 after learning about the cases from a doctor at Kantha Bopha Children's Hospital in the capital. Many patients first visited local health facilities before coming to the city, and investigators are now trying to piece together those reports to determine more about their conditions and what treatments were given, Asgari said.