Actually there are a few of them -- nowadays we seem to take HIV for granted, for example. But one of the biggest and most complex public health challenges facing the U.S. is not an infectious disease. It is of course obesity. Actually, that's a big of an oversimplification. Obesity is associated with too much caloric intake and not enough physical activity; obesity is itself a risk factor for problems such as diabetes, heart disease, and arthritis; and it is also a marker of diet and activity patterns which are direct risk factors. This analysis just published in NEJM gives an easy to understand summary of the impact. (Sorry, abstract only for you riffraff.)
The bottom line is that if current trends continue, the gains in health and life expectancy we are getting from reduced rates of tobacco addiction will be more than canceled out by the rising prevalence of obesity. If we could eliminate smoking and obesity altogether, 18-year-olds today would gain 3.76 years in life expectancy and more than 5 QALYs. (If you don't know what a QUALY is, here's an introduction.) That's a huge impact compared to the benefit, if any, from screening mammography; and much larger than the benefit from, say, universal flu vaccination, by orders of magnitude.
The most important takeaway, for me, is that this is essentially a social, political and cultural problem, not a medical problem. The only effective medical treatment for obesity is stomach reduction surgery, and while the benefits appear to outweigh the harms for some very obese people, that's hardly where we want to go. We need a radically different social environment -- a different food environment, first of all. The food environment is largely shaped by gigantic corporations that spend billions of dollars to sell processed foods that are cheap to make, addictive, and can be sold at a large profit. Children are exposed to thousands of commercials for junk food every year; there are fast food restaurants all over the city that sell cheap poison in the guise of food; and sugar water is served in schools. Public policy can change all that but only if the people go to war against Frito Lay, Coca Cola, CPC International, Kraft Nabisco and the rest of the dirty gang.
Agricultural policy is also essential to this struggle. We subsidize corn and soybeans, a lot of which goes into animal feed, much of the rest into junk food, but not vegetables. The subsidies go to giant agribusiness corporations, and not to small farmers who grow for local markets. The vast monocultures that our policy supports are environmentally and socially destructive as well as nutritionally toxic.
The issue of physical activity has a lot to do with TV and electronic entertainment, of course. Kids who used to go out and play baseball are sitting in front of X boxes. But it also has to do with land use and mass transit. Walkable communities built around transit hubs, with stores, entertainment and parks in walking distance of people's houses, have more active people. I live in one -- Jamaica Plain. I walk to the subway and back every day, I walk to the grocery store and the drug store and the neighborhood restaurants, and to Jamaica Pond, one of the world's great urban parks. That means not only healthier people, but less fossil fuel going out the tailpipe. But we subsidize highways and our mass transit systems are rotting.
These are political issues that go right to the heart of what kind of a world we'll have in fifty years. They are much, much more important than the latest bug that's going around.