Iodine deficiency.

Valentina Sivryukova knew her public service messages were hitting the mark when she heard how one Kazakh schoolboy called another stupid. "What are you," he sneered, "iodine-deficient or something?"

I think I'm going to start using this insult!

I always knew iodine was added to salt at some point for some health reason - but I was never sure actually why that was. It looks like iodine deficiencies can cause all sorts of nasty health problems:

Studies show that iodine deficiency is the leading preventable cause of mental retardation. Even moderate deficiency, especially in pregnant women and infants, lowers intelligence by 10 to 15 I.Q. points, shaving incalculable potential off a nation's development.
The most visible and severe effects -- disabling goiters, cretinism and dwarfism -- affect a tiny minority, usually in mountain villages. But 16 percent of the world's people have at least mild goiter, a swollen thyroid gland in the neck.

Clearly, this is a solvable problem and there has been some great success in eradicating this 'disease.' Especially in Kazakhstan.

In fact, Kazakhstan has become an example of how even a vast and still-developing nation like this Central Asian country can achieve a remarkable public health success. In 1999, only 29 percent of its households were using iodized salt. Now, 94 percent are. Next year, the United Nations is expected to certify it officially free of iodine deficiency disorders.
That turnabout was not easy. The Kazakh campaign had to overcome widespread suspicion of iodization, common in many places, even though putting iodine in salt, public health experts say, may be the simplest and most cost-effective health measure in the world. Each ton of salt needs about two ounces of potassium iodate, which costs about $1.15.

Here's the original NY Times article.