Selective toxicity, as the name implies, refers to the use of a chemical to selectively destroy a specific living tissue to the advantage of the host. The ideal chemotherapeutic agent would be one which is 100% fatal to the target cells in concentrations which are harmless to the host. No such agent exists, although some have a fairly wide margin of safety. More frequently, however, the therapeutic index is relatively small.
The greater the similarity between the target cell and the host cell, the greater the difficulty in achieving a good therapeutic index. Thus the anticancer drugs, which are acting against mammalian cells, are considerably more toxic than most antibiotics or anthelmintics. Potential mechanisms of selectivity are differences in permeability; differences in metabolic requirements of the target cell and the host cell (e.g., folic acid inhibitors); structural differences (e.g., bacteria have a cell wall, mammalian cells don't); differences in the rate of cell division (cell-cycle dependency - conceptually the basis of anticancer drug therapy); differences in drug target.
A high degree of selectivity does not guarantee safety because nearly all drugs have secondary effects not related to their principal mechanism of action.