The anti-malarial concentration window of selection . The drug concentrations (solid lines) following two doses which define the concentration limits of the window of selection are shown for patients with low parasitaemias (108/body). Total parasites within the body are depicted by dotted and dashed lines. The window opens when following the higher dose (blue) sensitive parasites (blue dotted line) are eliminated but de-novo resistant parasites (blue dashed line) can survive, and successful selection of resistance can take place. Following the lower dose the sensitive parasites can still survive (red dotted line) and later grow to outnumber the drug resistant parasites (blue dashed line) by >9:1, so there is no longer an opportunity for preferential survival of the resistant parasites and thus selection. The window closes. Concentrations between these two boundaries (the "window of selection") are selective.