Re: Worrying about my niece.
I wouldn't be concerned with her catching anything from doing this at all. I would be more concerned about why her bum is itching inside..if you see her scratching there a lot she could have worms which is quite common in children.
She cannot catch E coli or anything similar this way.
Re: Worrying about my niece.
Yes Annie is right Girl afraid. I remember one of my daughters did this and it was worms. She was very irritable too. Not saying your niece has them though.
Maybe just itching anyway-- worms can be treated easily.