Re: How does fluoride help prevent cavaties from forming (or kill the bacteria.)

Good question Francisco!  Fluoride is a naturally occuring mineral found in 
almost all food and water supplies.  The fluoride ion comes from the 
element Fluorine, which is never encountered in it's free state.  Fluoride 
is effective in preventing cavities or caries in several ways.    It makes 
the tooth structure stronger so that teeth can resist acid attacks (much of 
 what we eat are acidic to some degree, and also acid results from the 
plaque bacteria break down many of the sugars and carbohydrates in our 
diets).  Fluoride also remineralizes teeth or repairs them also preventing 
caries.  
Fluoride is available in most community water supplies. Or if you live in a 
 rural area and do not have "city water" there are fluoride rinses 
available to help protect your teeth.
If you'd like more information this is available online from the American 
Dental Association - Flouridation 
and Flourides.