Drug resistance in parasites: can we stay ahead of the evolutionary curve?

The articles in this special issue of Trends in Parasitology document the current status of drug discovery in various helminth and protozoan parasitic infections. Parasitic diseases present a unique challenge to those who try to prevent or treat them. In most cases, the parasite has evolved to evade the human immune system, so the human host can control, but not eliminate, the parasite. Design of effective vaccines against these diseases presents daunting difficulties; therefore, drugs are currently the only way to prevent or treat parasitic diseases. Under these circumstances, selection of resistance to any effective, well-tolerated drug is inevitable; the question is not if, but when. The goal of this review is to try to draw general conclusions about the measurement and selection of resistance to drugs directed against a variety of very different parasites.