In common usage, an antibiotic is a substance or compound that kills bacteria or inhibits their growth. Antibiotics belong to the more common group of antimicrobial compounds, used to treat infections caused by microorganisms, including fungi and protozoa. Most antibiotics are now semisynthetic, thanks to medicinal chemistry. They are modified chemically from original compounds found in nature. In addition to this origin-based classification into natural, semisynthetic, and synthetic, antibiotics may be divided into two broad groups according to their effect on microorganisms: those that kill bacteria are bactericidal agents, and those that only impair bacterial growth (also known as bacteriostatic agents).