Selective toxicity is an old phenomenon but adequate knowledge about it is slow in accumulating. Fundamentally it is the attribute of a substance or drug that makes it more toxic to a parasite than to the host. When that is a function of the dose, as in the case of antimony compounds for example, the same substance will kill or damage both—it merely takes more to kill the host perhaps due to his greater mass and size. The ideal is to kill or injure one organism without hurting another. The problem of selective toxicity is not limited to man but also refers to the same phenomenon in lower animal life and in plant life with regard to economic poisons.
There are many factors which have to do with the toxicity or activity of a drug. Substances which are quite similar, chemically, may be due to one or more of the