In tracing the course of preventive medicine, it is important to remember that at every stage, human necessity activated by fear and panic, was urging the adoption of preventive measures. There is a close connection between ancient knowledge of antiseptics and chemical agents aimed at preventing disease. Both Hebrews and Greeks employed antiseptics in their religious rites. Destruction of bedding and apparel and even of dwellings was practised in some pestilences. Chemical purification of rooms in smallpox is also valuable. Disinfection by heat in the form of steam has become an established practice in disinfecting contaminated bedding, clothing and woollen goods, as in smallpox and anthrax, or in disinfestation of these from lice. The Great War brought out the momentous importance of disinfestation as a means of preventing disease and increasing military efficiency.