[manmal]

Not to refute your n=1, but reviews are a safer way of establishing whether something actually works. Eg here, from 2015:

> There is insufficient information to determine whether initiation of a water fluoridation programme results in a change in disparities in caries across socioeconomic status (SES) levels.

> No studies that aimed to determine the effectiveness of water fluoridation for preventing caries in adults met the review's inclusion criteria.

> Over 97% of the studies were at high risk of bias and there was substantial between‐study variation

From here: https://www.cochranelibrary.com/cdsr/doi/10.1002/14651858.CD...

TLDR: The existing research is low quality.

[rixary]

Is caries the Spanish word for it or is that actually an English word for cavity as well?

[manmal]

It's originally a Latin word and, as many other Latin words, is now a medical term in English.