Medical science advanced rapidly in the second half of the 19th century, applying the exciting discoveries of a new science, bacteriology, which transformed public health. The great bacteriologists of the late 19th century identified many pathogenic bacteria, classified them, developed ways to cultivate them, and, most important, worked out ways to control their harmful effects, using sera, vaccines, and "magic bullets" such as the arsenical preparations that Ehrlich developed to treat syphilis.