18 year old HS senior has the crud, started Friday, tested for covid Sunday night and now this morning, both tests negative. Mostly nose, now his throat is sore probably from all the drainage. I know other stuff is going around and am confident it is something else.

I am fine, no symptoms.

I am the primary caretaker for my MIL who just had chemo last Wednesday Aug 18th. Am I any danger to her? I know her immune system has taken a hit and will hit a low point this week. She has a doc appt tomorrow, I am her driver and plan to accompany her into the doc appt.

Not a doctor, but based on my training as a lifeguard for many years, lifeguard instructor and volunteer firefighter, always take reasonable precaution to prevent disease transmission. I would wear a good fitting N95 mask (the type without a vent) around her. I would also have her wear one. Wash hands often, before and after you touch your face/pick your nose/eat/etc. It's probably a good idea for your son to do the same and for him to quarantine as much as possible. Let him play video games in his room all day and order him a pizza.