Bacterial diseases have played a dominant role in human history. Widespread epidemics of cholera and plague reduced populations of humans in some areas of the world by more than one-third. Bacterial pneumonia was probably the major cause of death in the aged. Perhaps more armies were defeated by typhus, dysentery, and other bacterial infections than by force of arms. With modern advances in plumbing and sanitation, the development of bacterial vaccines, and the discovery of antibacterial antibiotics, the incidence of bacterial disease has been reduced. Bacteria have not disappeared as infectious agents, however, since they continue to evolve, creating increasingly virulent strains and acquiring resistance to many antibiotics.
Well one good example is the production of insulin using bacterial plasmids. The human insulin gene is spliced into a bacterial plasmids. The bacteria are allowed to divide in a vat and reproduce, with the gene activated to allow them to produce human insulin. The bacteria are then killed off, and the insulin separated and purified