A study says more than a third of malaria-fighting drugs tested over the past decade in Southeast Asia and sub-Saharan Africa were either fake or bad quality.

The study published Tuesday says bogus and badly made drugs are threatening to upend a decade of progress fighting the mosquito-transmitted disease.

Fake drugs can lead to deaths because they contain no malaria-fighting agents. Pills without enough of the active ingredient to kill all malaria parasites are problematic because they increase drug resistance. That means malaria eventually will outsmart medicines and render them useless.