The original misunderstood health issue, before vaccines or GMO's caused people who knew nothing about science to make proclamations about public health, was Fluoride. Yet it has been incredibly beneficial. Let's get into it.
Fluoride is a form of Fluorine, the 13th most abundant element on Earth. It's naturally found everywhere, from most water sources to the vegetation in which is accumulates. Fluoride in water is a result of it's general abundance on Earth, and fluoride in food is a result of bio-accumulation.
Like anything, too much isn't good, but all the sources you're likely to come in contact with contain very low levels. It's occurrence in nature runs the spectrum; in Tanzania, the alkaline volcanic activity associated with the geologic process of the rift valley causes fluoride levels in water to be dangerously high, and negatively impact human health. There are places on Earth which have optimal levels for human health, benefiting about 50 million people globally. For most of us living in cities, it's added to water, which. In some countries it's added to food (like iodine), but in the United States, the maximum content is regulated to ensure it's not above safe levels.
Avoiding Fluoride entirely is not possible. Nor is it desirable (we'll go into detail here). Whether it is harmful or beneficial is a matter of quantity. If you're lucky enough to live in the United States, chances are you're getting the right amount to protect your teeth while having no negative impact on your health, because municipalities without enough will add, and if there's too much, it they're required to lower the levels. However, wells are not regulated, so if you have a well and are concerned, get your water checked.
Understanding the science around these sorts of things will save you a ton of worry. Fluoride is natural, and really quite necessary for optimal health. Don't let people who are uninformed scare you. Direct them here, or to other resources which are will provide some clarity and quell the baseless rumors. For those that really want to get into it, check out what the WHO has to say.