One of the most striking correlations in all of public health is the strong association between education and health status. The more education people have, the better health they seem to have. For a striking graphical look at the phenomenon, the CDC's MMWR this week has a great chart on self-reported health status and educational status. "Association does not mean causation" is a mantra of epidemiology, though - meaning that just because there's a link that doesn't mean more education causes improved health. It could be any number of confounders. Basically it's a chicken/egg thing: Are the better educated likely to have better health because they're in school - or are the more healthy simply more likely to get an education? For health economists, this is an endless source of chatter and speculation. They can look at all sorts of side effects: Are the better educated more likely to learn about sound health practices (ie, don't smoke)? Are the better educated more likely to earn more, and therefore be able to buy better health?
That subtlety between cause and effect is usually missed, though; the New York Times did a big front-page story in January headlined "A Surprising Secret to a Long Life: Stay in School." (The story is stuck in the paid-only archives now, so I'll omit a link).
Ultimately, of course, it may not matter what's behind the connection. If going to school teaches you to live healthy, great. If healthy people are more likely to stay in school, great. Either way, you'll end up healthy and wise - if not necessarily wealthy, too.