# Many serious events in 1970'S highlighted the need for better understanding EM such as: a)Emergence of a serious new waterborne food borne pathogens that posed a threat for human & animal health. b) an increase in waste disposal leading to contamination of surface water & ground water supplies with organic & inorganic chemicals. c) The development of new technologies in detection & identification of micro-organisms such as PCR. d- discovery of the resistant to antibiological agents(antiseptics & antibiotics ). e- Recently discovered techniques that have a significant effect on human health such decrease of cholera and typhoid fever by applying filtration and use of disinfectants