Ever since the 1970's the United States, along with several other countries, has been putting fluoride in the public water supplies in an effort to reduce tooth decay, and ever since there has been controversy. While some of the controversy concerns some legitimate reasons, such as cost, and effectiveness, and some safety concerns. Others concerns tend to be just outright conspiracy theories.
The two main conspiracy theories concerning water fluoridation is that the fluoride in the water is causing peoples' intelligence to lower, and that it's causing people to become infertile. The reason why conspiracy theorists believe this is because fluoride is a poison, and that any amount of fluoride, no matter how small the amount, is dangerous to humans.
First, if water fluoridation did cause infertility, then why hasn't the birth rate and population gone down? Also, the fact is, is that poisons don't infertility. Genetics, disease, radiation, and injuries to reproductive organs causes infertility.
Second, while our intelligence "might" be lowering (and this is an issue of debate) it's more likely the result of the lowering of educational standards, and possibly some peoples' distrust in science.
It is true, fluoride can kill a person, but only if it's consumed in a large enough amounts. The amount varies from person to person, but usually it's somewhere between 5 to 10 grams.
It would be impossible to consume even a fraction of fluoride through drinking water from public water supplies to cause a person any harm. This is due to the fact that the water has diluted the fluoride so much that it renders it to the point where it is harmless.
It should also be noted that fluoride, even in larger, but non-lethal amounts, does not cause infertility, or lowering of intelligence. It can cause leg pain and incomplete stress fractures, and also stomach irritation and sometimes even ulcers. The amount of water you would need to drink at one time in order to cause the leg and stomach problems would be so much that it's physically impossible to do. You would either be physically unable to continue to drink, or you would dry drown.