Chemotherapy was founded on Paul Ehrlich's hypothesis that protozoal infections may be treated with medications that only react with the protozoa's target tissues, which could be an enzyme, DNA, RNA, or even a structure with an unknown constitution. The ideas that underpin chemotherapy are highlighted in this text. Groups of medications that are now prescribed serve as excellent examples of these concepts. Our goal is to present a paradigm for fundamental drug design that clarifies the evolution of medications used in clinical settings.