What is meant by "selective toxicity"?

According to Todar's Online Textbook of Bacteriology, selective toxicity refers to the ability of antibiotics to selectively target bacterial cells yet have no toxicity towards human cells. Most selectively toxic antibiotics target bacterial cell wall synthesis, bacterial nucleic acid synthesis or bacterial protein synthesis.

An antibiotic's therapeutic index, which measures selective toxicity, is the ratio of how much is toxic to a human versus how much is required to defeat an infection, according to Todar's Online Textbook of Bacteriology. Selective toxicity measurements refer only to human cells; the effects on normally present bacteria, such as those in the intestines, are measured separately.