It's not bird flu or SARS, and nor does it appear to be contagious, but little more is known about a mysterious disease that has killed dozens of Cambodian children, some within 24 hours of being hospitalised.
Medical experts are scrambling to respond to what the Cambodian health ministry and World Health Organization (WHO) have labelled an "undiagnosed syndrome" that has claimed the lives of at least 56 boys and girls, mostly toddlers, since April.
Officials said just one child was believed to have survived the illness and the high fatality rate has spread concern among Cambodians, 30 percent of whom live below the poverty line according to the World Bank.
The WHO has put neighbouring countries on alert about the killer disease, that starts with a high fever and is followed by respiratory and neurologic symptoms "with rapid deterioration of respiratory functions".
There have been no cases reported outside Cambodia so far.
"We are looking at detailed information from the hospital records and analysing each and every case. We hope to have a better picture in the coming days," said Ly Sovann, deputy director of Cambodia's Communicable Disease Control Department in a joint statement with the WHO on Friday.
Paediatrician Beat Richner, the founder of Kantha Bopha children's hospitals, which see around 85 percent of Cambodia's severely ill youngsters who make it to treatment, was the first to raise the alert over the illness.
The Swiss doctor, who told Cambodian health officials about the illness last month, gave a higher toll than the WHO, saying 64 children had died from the disease since mid-April, while two more had recovered.
The victims were aged seven and under with most being between two and three years old, Richner told AFP in an interview. The most recent death was on Saturday.
"All these children have encephalitis (inflammation of the brain) and in the later hours of their life they develop a severe pneumonia with a destruction of the alveoli in the lungs. That is the reason they die," he said.
The alveoli, or air sacs, are pockets in the lungs where the exchange of oxygen and carbon dioxide takes place.
"We think it's either a virus, an intoxication, or both," Richner said.
While it is impossible to rule out contagion at this early stage, Richner said he had yet to come across two cases in the same family, and no health workers appear to have fallen ill after caring for the patients.
The WHO has also said it has found no clusters, though most of the patients came from central and southern parts of Cambodia.
Like the WHO and the health ministry, Richner's staff are racing to find the cause of the disease, sending blood and tissue samples to the Institut Pasteur -- a renowned infectious disease research centre.
Early results from a selection of those samples show some of the children had been infected by a lethal strain of hand, foot and mouth disease, although Richner said more analysis was needed.
He said all the patients who died were treated in private clinics in their local areas before being brought to the Kantha Bopha hospitals in the capital and the northwestern province of Siem Reap.
"They all got injections or infusions by private carers before coming to us," he said. "Some died four hours after arriving."
From his own figures, Richer said the two patients that lived were treated only by Kantha Bopha staff, suggesting that botched medical treatment may be a factor.
The WHO said it was too soon to draw conclusions.
"We are looking at the possibility of this being something new, a collation of different diseases with similar clinical presentations but caused by a different pathogen," said doctor Nima Asgari, a public health specialist at the WHO office in Cambodia.
The UN health body and Cambodian officials have urged parents to bring their sick children to hospital if they see any signs of "unusual illness".
Joining the daily queues of hundreds of families seeking treatment at Kantha Bopha, In Sitha said she first heard about the mystery illness while her three-year-old son was in hospital with pneumonia last month.
"I heard it develops very fast," she said. "As a mother, I feel very scared and helpless about this. I just hope this unknown disease can be cured so that my son and other people's children can survive."
Richner urged parents to stay calm saying "there's no reason for hysteria," and pointed out that an ongoing dengue fever epidemic was a much larger worry.
In June alone, more than 5,000 children were hospitalised with haemorrhagic dengue fever, compared to just 34 cases of the unknown disease. "That's the big problem," Richner said.
Explore further: Cambodian girl dies from bird flu: WHO