Ten days ago, Chhem Bunchhin, a teacher in Battambang Province, became ill with chills, fever, headache and vomiting.

At a nearby health centre he was treated with drugs considered a "silver bullet" in the battle against falciparum malaria.

This treatment with artesunate drugs was part of a clinical study being carried out by the US Armed Forces Research Institute of Medical Science (AFRIMS).

In the past, artesunates have always cleared malaria parasites from the blood in two or three days. But after four days of monitored treatment, Chhem Bunchhin was still testing positive for parasites....

BBC NEWS | Asia-Pacific | Fears for new malaria drug resistance