Story highlights Seventy-four cases have been identified, Cambodia's Ministry of Health says

Surrounding countries notified of disease; doctors on alert in Hong Kong

Children suffered swelling in their brains and died because lungs failed

The World Health Organization and Cambodian health officials are investigating why at least 61 children in the country have died mysteriously after suffering severe neurological and respiratory complications.

Dr. Beat Richner of the Kantha Bopha Children's Hospitals, who first alerted Cambodia's health authorities about the unknown disease, said that as of Friday he knew of 64 cases in which only two children have survived.

The Cambodian Ministry of Health said that 56 of the deaths were preceded by a common syndrome of fever and respiratory and neurological problems.

Seventy-four cases of the disease have been identified, the ministry said.

Countries surrounding Cambodia were informed of a deadly disease that killed dozens of children this week through the International Health Regulations event information system, which provides public health communications.

In Hong Kong, a major air hub in the region, health officials responded by alerting doctors to be watchful for patients returning from Cambodia who have respiratory symptoms. Travelers who have been to Cambodia were told to visit their doctors if they developed respiratory symptoms.

A majority of the identified cases were in children under 3, the health ministry said.

"The investigation is ongoing. We are looking at detailed information from the hospital records and analyzing each and every case. We hope to have a better picture in the coming days," said Dr. Ly Sovann, a deputy director at the ministry.

The children who fell ill first experienced a high fever followed by respiratory problems. Some of them also had neurological symptoms that included convulsions, according to the WHO.

Richner said the patients suffered from encephalitis, which is the inflammation of the brain.

"They are hospitalized," he said. "They arrive in our hospital; in the last moments ... they die because their lungs are destroyed."

When asked what he thought caused the deadly illness, he said: "I think our idea is an enterovirus or an intoxication of a drug," or a combination of both.

Richner added that the number of cases affected by the unknown disease is low -- 34 cases in June, compared with the 75,000 sick children at Kantha Bopha's outpatient clinics and 16,000 hospitalized kids.

The majority of the cases came from the southern part of the country, but there haven't been signs of clustering, according to the WHO.

The young patients were brought to hospitals in the capital, Phnom Penh, and the northern tourist hub of Siem Reap -- the two biggest cities of Cambodia. The Angkor Hospital for Children in Siem Reap reported one case of the mysterious illness as of Friday, according to an e-mail received by CNN from Dr. Bill Housworth, the hospital's executive director.

"WHO supports the Ministry with the investigation and detailed analysis of each of these cases to find out the cause," wrote Dr. Pieter Van Maaren, a representative of WHO in Cambodia in an e-mail Friday.