DDT is an acronym that stands for dichloro-diphenyl-trichloro-ethane; it is a type of pesticide that helps prevent the spread of disease by insects. DDT has a tendency to accumulate in an animal's fatty tissues. This accumulation has marked effects on the organisms and the animals that consume them.
Bald eagles consume many fish due to their size, and were poisoned by the large amounts of DDT in their food supply. This led to the bald eagle laying thin eggs that were unlikely to survive, and their population plummeted. DDT is not commonly used unless there is a widespread threat of insect-borne illness, such as malaria.