Microbial pathogens are microorganisms that are capable of producing disease. Virtually all groups of bacteria have some members that are pathogens. One notable exception is the Kingdom Archaea, where there are no known pathogenic members. Other disease-causing microbial agents are viruses and parasitic protozoa. Earlier methods of detecting and identifying microbial pathogens involved culturing and isolating bacterial colonies in growth media in the lab. With the advent of polymerase chain reaction (PCR) assays, identification of microorganisms that were difficult or impossible to culture became possible. Many microbial pathogens can be controlled with antimicrobial drugs called antibiotics. However, these drugs are not effective against viruses or parasites, and indiscriminant use may cause resistant strains of pathogens to evolve. Antibiotic resistance has resulted in the reemergence of several disease, and as of 2002, most of the major bacterial diseases that infect humans are becoming resistant to antibiotics.