Is there any proof that diarrhea is due to the lack of toilets? Although septic toilet is still a toilet, I don't think it's orders of magnitude more hygienic s

Is there any proof that diarrhea is due to the lack of toilets? Although septic toilet is still a toilet, I don't think it's orders of magnitude more hygienic solution for a densely populated area. It requires maintenance and cleaning, and in an area with heavy rains, it could be a disaster (according the article, it's concrete one that can easily overflow). I think they should address the running water and soap before tackling the toilet issue!