Caitlin Rivers' post on the "public health paradox: why people don't get flu shots" hits the nail on the head:
Unfortunately, the root of this problem is deep. The problem is that when public health works, it is invisible. It's an insidious, persistent public relations issue that plagues public health. Nobody sees when a chain of disease transmission is broken, or when contaminated food is prevented from reaching the market, or when toxic pollutants don't enter the environment. That's the point: the goal of public health is prevention, not reaction....What then can be done to counteract these misperceptions? First, public health needs to be more vocal about its successes. This graphic of crude death rates for infectious diseases during the 19th century, for example, should be widely disseminated. A little self-promotion could go a long ways.
That's one reason I like Millions Saved, from the Center for Global Development -- it highlights "proven success in global health." One of the things that struck me when reading it was that most of the people who benefited from these interventions and programs would have no way of knowing that they benefited.
For another positive take, check out Charles Kenny's book Getting Better.