The mosquito used to be thought of as a mere nuisance until, in Victorian times, it was revealed that they were the ultimate killing machine, spreading the sometimes fatal disease of malaria. A century on, Horizon traces the astonishing war on the insect that this discovery unleashed, and how the mosquito and malaria have returned, increasingly immune to man's chemical weapons and spreading once more. Determined and seemingly unstoppable, together mosquitos and malaria are nature's deadliest partnership.