My 9 year-old cat, Heather, was absolutely fine until last Tues AM (Nov.2)
Instead of meowing at my bedroom door and waking me up as she usually does,
I found her laying on the couch, looking ill.  I took her to the vet WEd, he
wasn't impressed,thought it was a virus.  Thurs. AM she was decidedly worse,
hadn't eaten in two days, and breathing rapidly.  This time, the vet did a
chest xray, and it appeared she had a right pleural effusion (collection of
fluid between chest wall and lining of lung.)

He drained some of it out and sent it for cytology;the path was
inconclusive, but he said in a cat this age he worried about cancer.  I'm in
the medical profession (physician assistant) and I work in oncology.  And I
know pleural effusions in humans can mean cancer or infection.

ANyway,
yesterday morning, her breathing was much better, and today her meow was
much stronger and she was more active .  However, though she is taking
water, I have yet to witness her eating.  She is using the litter box and
grooming.

I don't want to delude myself, but has anyone else here have any experience
with this?  I don't know what to think.  Could it possibly be an infection
or pleuritis?  I have searched the web, but am frustrated with the paucity
of veterinary info available as opposed to human.

Thank you so much for listening.  I am extremely upset about the possibility
of losing my beloved pet, though I know she is older and I have to face it
eventually.