Many years ago the people of a town in South America had a big problem. Their cats were dying, and no one knew the reason. Not long before, every house had its own cat--sometimes even two or three. They killed the mice that came into the town from the forests, but suddenly the cats were dying. It was very strange. First the cats began to shake all over. Then they stopped eating, and after a few days they died. The people didn't know why so many cats were dying.
Like other areas in the country, many insect pests were found in this town. These pests often carry dangerous diseases. But at that time there were no insect problems in the town, because people were using a new poison called DDT to kill insects. They thought DDT was a very useful poison. It did a good job of killing insects. Several times a year they spread DDT in every house in the town, so they thought that the diseases carried by insects were not killing the cats.
Soon a strange thing began happening in the town. The cats were dying, and a lot of mice appeared in the town. Then some of the people suddenly became very ill. The doctors discovered that it was a disease called black typhus. The doctors knew the mice were carrying black typhus. So people caught all the mice they found and killed them. After that black typhus disappeared from the town.
Did the cats also die because of black typhus? The doctors did not think so, because they found that cats didn't have black typhus.
A team of doctors came to the town and began to study why the cats were dying. One of the doctors in the town still had a few dead cats' bodies. At last they found there was enough DDT in the cats' bodies to kill them. The DDT spread in the house went into the cats and killed them.
Now the doctors knew everything: At first the mice couldn't come into the town because many cats lived there. Then DDT was spread in the town to kill insects, and the cats began to die. After all the cats were dead, there was nothing to keep the mice out of the town. Hundreds of mice came into the town. The black typhus disease the mice carried was spread through the town's food and water. Soon the people became ill and died.
The scientists invented a wonderful poison that killed insect pests. But these scientists forgot that everything in this world is connected. The poison that kills insect pests also kills cats, birds and fish. By solving one problem, people sometimes create new and more dangerous problems.