You know already that lead is bad for you. This is why we have such a thing as "unleaded" gasoline, and in general don't allow lead near anything you're about to breathe or eat. So what does this have to do with chocolate?
Well, it just so happens that the shells of cocoa beans, the chief ingredient in chocolate, have an almost supernatural ability to absorb lead from the air. Of course, this is only a big deal if there's a lot of lead in the air, right? Sure, which is why it's unfortunate that Nigeria, where a shitload of our cocoa comes from, still has lead in their gasoline. That's why a Hershey bar tastes so damn good: The secret ingredient is African exhaust.
The average candy bar ranks fourth for highest lead content in a food. Don't panic yet, though; lead occurs naturally in everything, even wholesome vegetables and grains, and while there is no amount of lead that isn't harmful to the human body, a little bit of the stuff in our bloodstream is more or less unavoidable and probably won't do anything bad enough that we'll ever notice.
Infants and children, on the other hand, are basically screwed. Because they're already stupid, kids absorb higher concentrations of lead into their system than the rest of us, just through their day to day activities of eating dirt and inhaling paint chips. Also, they breathe faster than an adult, which further draws in whatever lead happens to be floating around and sends it toward their sensitive little brains. Even before they start wailing on that candy, they have more lead in their bodies than Vito Corleone. Then comes Halloween.