Malaria is a disease that is caused by the eukaryotic parasite Plasmodium and is transmitted between individuals by the bite of infected mosquitoes. Different species of Plasmodium cause disease in different vertebrate species, but in all cases the parasite infects and destroys red blood cells. The repeated emergence of drug-resistant strains of Plasmodium parasites responsible for human infections has caused researchers to continually search for new drugs or combinations of drugs that can provide an effective treatment. A drug that has proven effective in recent years is artesunate, but now artesunate-resistant Plasmodium strains have also appeared. Researchers investigated whether artesunate-resistant Plasmodium strains could be effectively treated by other drugs. In a mouse model of malaria, the researchers determined that a particular strain of artesunate-sensitive Plasmodium (ART-S) could be largely eliminated from the mice at a dose of 0.15 mg/kg. By repeated exposure of the artesunate-sensitive strain ART-S to increasing concentrations of artesunate, the researchers developed a Plasmodium strain that is highly resistant to artesunate (ART-R) and requires a minimum dose of 240 mg/kg to eliminate the parasites. The researchers then compared how effectively several common antimalarial drugs eliminate the ART-S and ART-R strains in vitro. The researchers infected human red blood cells with the ART-S and ART-R Plasmodium strains and incubated the infected cells in tissue culture medium. They added serial dilutions of the drugs to replicate cultures and determined the IC50 for each drug, the concentration of the drug that reduced the parasite load in the red blood cells by 50% (Table 1).