This chlorinated organic insecticide was discovered by Swiss chemist Paul Muller in 1939. DDT has been especially useful in controlling mosquitos that carry malaria, but some strains of the insects have become resistant to it. DDT has comparatively low acute toxicity in humans (it is thought to cause cancer), but it persists for a long time in the environment and is disastrously toxic to birds, especially top-level predators such as hawks and eagles (see biomagnification). The chemical interferes with birds' ability to metabolize calcium, and thus affected birds lay eggs with fatally thin shells. DDT has been banned in most countries, but it is still widely used on crops in Latin America.