Different levels of anti-malarial drug resistance provide different probabilities of individual parasite survival (inset). The total numbers of parasites in an individual patient change with time. Treatment failure is defined as a failure to clear detectable parasites from the blood (R3, R2) or a subsequent recrudescence (R1). The former WHO classification of resistance translates into growth rates per cycle (PMF) as follows; R3; >0.25, R2; 0.1 to 0.25, R1a; 0.1 to 0.01, and R1b; <0.01. The inset shows the corresponding probabilities of an individual parasite surviving one cycle if exposed to therapeutic drug concentrations.