Strains of drug-resistant malaria are becoming more dominant in Vietnam, Laos and northern Thailand after spreading rapidly from Cambodia.
Malaria is caused by parasites which are carried by mosquitoes and spread through their blood-sucking bites.
And scientists have discovered a growing number of cases where the parasite has mutated making it resistant to drugs.
“We discovered (it) had spread aggressively, replacing local malaria parasites, and had become the dominant strain in Vietnam, Laos and northeastern Thailand,” said Roberto Amato, from the Wellcome Sanger Institute.
Malaria can be successfully treated with medicines if it is caught early enough, but resistance to anti-malarial drugs is growing in many parts of the world, especially in south east Asia.