Kitten health question

I just recently adopted a kitten, it was found on a highway by a kind woman. The kitten is female, around 5 weeks old. She has had a stool test, deworming, along with other treatments and has a clean bill of health except for an ear infection which is being treated.
My question is should i be worried about the kitty’s health if she has been having diarrhea for a few days now (i think, previous foster didn’t specify but did mention it) And should i be worried about the kitty getting my other cat (female 5 years old) sick aswell?