How do you remind a demented individual or make her understand the reasons for hygiene and safety precations?

You can't : Dementia is a horrible illness which slowly (or rapidly) removes the victims ability to understand and communicate. The best you can do is be supportive.

Avoid judgement: Try to avoid asking the person with dementia why they didn't do something, instead focus on getting the task done. If they haven't taken a shower don't ask why or try to explain the importance just say ok it's time to take a shower. Of course this doesn't always work but may help to build habits if you are consistent.