Anti-infective agents are medications that work to prevent or treat infections, including antibacterials, antivirals, antifungals and antiparasitic agents. Anti-infective agents are microbial metabolites or synthetic analogues inspired by them that, in small doses, inhibit the growth and survival of microorganisms without serious toxicity to the host. Selective toxicity is the key concept. Antibiotics are among the most frequently prescribed medications today, although microbial resistance resulting from evolutionary pressures and misuse threatens their continued efficacy. In many cases, the clinical utility of natural antibiotic has been enhanced through medicinal chemical manipulation of the original structure, leading to broader antimicrobial spectrum, greater potency, lesser toxicity, more convenient administration, and additional pharmacokinetic advantages. Through customary usage, the many synthetic substances that are unrelated to natural products but still inhibit or kill microorganisms are referred to as antimicrobial agents instead. Because of a significant decrease in the pace of novel anti-infective discovery, increased regulatory constraints, and greater profits to be made by the use of medications for chronic conditions, there is presently a decreased research emphasis on antimicrobial agents. This coincides with a dramatic increase in microbial resistance to chemotherapy, that portends a bleak future in which humankind may once again face infectious diseases with few available countermeasures.