To exterminate a living species by accident is normally frowned on. To do so deliberately might thus seem an extraordinary sin. But if that species is Plasmodium falciparum, the sin may be excused. This parasitic organism causes the most deadly form of malaria. Together with four cousins, it is responsible for about 450,000 deaths a year, and the ruination of the lives of millions more people who survive the initial crisis of disease. Besides the direct suffering this causes, the lost human potential is enormous. The Gates Foundation, an American charity, reckons that eradicating malaria would bring the world $2 trillion of benefits by 2040.