Let me make some sense of that first paragraph before you give up in
frustration:

We have a 6 yr old tabby who was recently introduced (February) to a new
Pnut is the 6 yr old tabby.

the cold or upper respiratory infection. The new cat completely
recovered and is fine now, and never had a problem to begin with.
The new cat _did_ have a problem to begin with, it just wasn't much of a
problem and had no life-threatening complications along with it.

I forgot to mention that Pnut grinds her teeth loudly on occasion and
especially frequently before vomiting. I assume this is just a stress
factor but I feel it is worth mentioning. She also seems to have
significantly more drainage from her eyes than normal, but that may be
due more to poor grooming than anything else.