Bacteriophages were initially considered to offer the key to the control of bacterial infections; early studies, however, proved largely unsuccessful. In the 1940s and 1950s, pioneering studies into the structure and physiology of host/phage interactions laid the basis for the development of molecular biology and a spectrum of new biotechnologically-based industries. Bacteriophages able to infect most procaryotic groups of organisms have been isolated, and are readily isolated from soil, water, and sewage and most environments colonised by bacteria. Ecologically, phages are as varied and as versatile as their hosts with some able to survive extremes of temperature (up to 95 degreesC) and extremes of pH as low as pH 1.