D.D.T is an abbreviated term. The full name of D.D.T is Dichloro diphenyl trichloroetheane. It is very effective as preventive medicine against yellow fever, plague and malaria. Dr. Muller of Switzerland was awarded nobel prize in Medicine in 1948 for discovering insecticidal property. DDT was extensively used in many countries for few years after its discovery, However, later on it was detected that some chemicals including DDT, dieldrin chloroden etc. can be as harmful as to cause cancer.
In california in the USA there is a lake named "Clear". Once DDT was sprayed to clear out a type of fly from the lake. On doing that they found that not only flies but also many birds died. This was because DDT got into the body of the birds through food chain. However, it is to be noted that human body can endure a certain amount of DDT and dieldrins. Thus DDT is both useful and harmful at the same time.