In the 50s and 60s people began to use a lot of pesticides on plants and trees. Pesticides kill insects that eat vegetables and fruit; if the insects die, you get more vegetables and fruit.
But where do all these pesticides go? Into Eart, the rivers, the sea, the air, the birds, animals and insects - and some into your food. Are pesticides good for you or bad you? Does it matter?
In Borneo in the 1950s, mosquitoes were giving people a disease called malaria. So the government used a lot of pesticide to kill the mosquitoes. Soon there was less malaria - and that was a good thing.
But then grass roofs started falling on people's heads because the caterpillars started eating the grass. Before, wasps ate caterpillars but the pesticide kills the wasps too. Now there was nothing to stop the caterpillars.
Cats ate the animals that ate the wasps, and without food, the cats began to die. And with fewer cats, there were more rats. The rats made people ill, and so people were still dying.
Now they had a worse problem than before because people were dying because of the rats than from malaria - and that is why 14.000 cats arrived by parachute in the villages of Borneo 1950s!