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.