SUBREDDIT: r/relationships
TITLE: My [24F] fiance's [26M] mother's house is causing our son to come back sick. How do I go about this?
POST: We've been together for four years and from the beginning, me and his mother have hated each other. When my son was a little over a month, she started screaming in my face and eventually kicked me in the shin.... Over laundry. Now, I've never been the type of person to have someone hit me in any type of way and just walk away like nothing happened so we ended up getting into a large physical fight and I haven't seen her since the fight. Before she kicked me, she called me a "trailer trash slut", "whore" and even threatened to take away my son. So after the fight, I didn't let her see my son for about a year. Of course, this really bothered my fiance and I ended up caving in and letting his mom see him. Well now three years have passed and I still haven't seen her since the fight. She just moved into her boss's house and every single time, my son comes back with an intense cough and runny nose. The other two times he was vomiting. He is only three years old so I can't ask him why he is coming back sick. So my question is, how can I go about this without upsetting my fiance? His response is to just give him allergy medicine because his mother recently got a cat. The allergy medicine isn't working and my fiance is defensive because he knows I hate her.
TL;DR: 
My son keeps going to his grandmothers house and coming home sick. I don't know how to respond to this since my fiance knows I hate her and is very defensive over her.