There are two ways to take anti-malarial drugs: the expensive way, which helps the world; and the cheap way, which helps only the patient. Most Africans cannot afford the expensive way and, as a result, the world's most effective anti-malarial drug may lose its potency.
That drug is artemisinin, available either by itself as a “monotherapy”, or with other drugs as a more costly artemisinin combination therapy, or ACT. For now, the monotherapy is excellent from the patient's perspective. Yet the ACT is greatly preferred by global health wonks, because monotherapies tend to encourage drug resistance in the malaria parasite.
This is no mere theoretical concern: malaria is now highly resistant to a previous wonder drug, chloroquine, and researchers have detected signs of resistance to artemisinin in areas where monotherapies are widely used.