Much of the region’s success in battling what was once a leading cause of death can be attributed to two-drug combination pills containing artemisinin, an inexpensive and effective drug invented in China decades ago.

But a new, drug-resistant strain of the disease, impervious to artemisinin and another popular drug with which it is frequently paired, piperaquine, threatens to upend years of worldwide eradication efforts — straining health care systems and raising the prospect that the death toll could increase again.

In recent years, public health officials have tracked the spread of deadly falciparum malaria parasites from western Cambodia to Thailand and Laos, and most recently into Vietnam.