ANTIMICROBIAL AGENTS can be used to treat infections by microbes (microorganisms), which include most important classes of pathogenic organisms — viruses, rickettsia, mycoplasma, chlamydia, protozoa, bacteria and fungi (though not helminths). Since bacteria are the largest and most diverse group of pathogenic microorganisms, antibacterials (mainly ANTIBIOTICS) form the major constituent group of antimicrobial agents. See ANTIBACTERIAL AGENTS; ANTIFUNGALS; ANTIPROTOZOALS; ANTIVIRAL AGENTS.