New drug-resistant strains of the parasite that causes malaria have been identified by scientists.
Parasites in western Cambodia have been found that are genetically different from other strains around the world. These organisms are able to withstand treatment by artemisinin - a frontline drug in the fight against malaria.
Reports of drug resistance in the area first emerged in 2008. The problem has since spread to other parts of South East Asia.
The study is published in the journal Nature Genetics.
"All the most effective drugs that we have had in the last few decades have been one by one rendered useless by the remarkable ability of this parasite to mutate and develop resistance,'' said Dr Olivo Miotto of the University of Oxford and Mahidol University in Thailand.
A BBC science reporter said parasites have developed a resistance to a succession of malaria drugs since the 1950s. The problem has spread to other parts of Asia and Africa.
Now scientists are worried the same thing will happen with artemisinin. This drug is used widely around the world against the mosquito-borne disease and can treat an infection in a few days when it is used in combination with other drugs.
To investigate, scientists sequenced the genomes of 800 malaria-causing parasites (Plasmodium falciparum) collected from around the world.
"When we compared the DNA of the parasites in Cambodia, they seem to have formed some new populations that we have not really seen elsewhere," Dr Miotto said.
The international team has found three distinct groups of drug-resistant parasites present in the area.