The Consequences of DDT

Although DDT reached its peak in the 1960s, it was first produced by the US during WWII for use on its soldiers to get rid of head and body lice, as well as malaria and typhus. 

One effect:

When DDT began being produced in mass quantities, its effects were not noticed until they became very severe. The pesticide gradually accumulated in small prey, and due to "bio-cumulation" or the mass collection of nutrients from pray being absorbed into the fatty tissue of bigger animals, large creatures like whales and dolphins were harshly affected. The concentration of DDT built up in their fatty tissue was way beyond the levels of it in the outside environment.

Another effect:

There was also an issue with egg thinning, or the deterioration of the structure and integrity of eggs of some animals like birds, severely hurting their population.

DDT was a strong pesticide with a lengthy half-life, meaning that it lingered in the environment for a long time after it was applied. There were actually traces/effects of the chemicals that had spread all the way to the Arctic and Antarctic. This was because they were spread by air and water, largely enabling them to spread far and wide. Many parts of the world were DDT spread were not known about until its effects were too big. The use of DDT had not been monitored closely enough before it was too late.