The pesticide dichlorodiphenyltrichloroethane was invented in the 1940s and was used after the Second World War in huge quantities across the world to control malaria-carrying mosquitoes. Gradually it emerged that it was disrupting the hormones of birds - particularly birds of prey - causing them to lay thin-shelled eggs which often didn't survive intact. In some parts of the world, such birds were rapidly reducing in number (there's no hard evidence that DDT directly harms humans).
Pressure from environmental groups in developed countries reduced the use of DDT, which is now effectively banned - only a few countries continue to use it. However, the rate at which its use has declined has been precisely in step with a resurgence in the incidence of malaria, which now kills a million people a year, most of them children and pregnant women.
This raises a classic question of balance of risk and benefit. DDT is cheap and effective. But it has a lethal effect on one part of the eco-system.