Could there be something in my house?

I am starting to wonder if there could be something in my house that makes my cats sick. I have never owned a healthy cat in this house, with the exception of my one year old male kitty that I have now (knock on wood). He did have a problem with loose stools for several months when he was a kitten though. I have mentioned my IBD kitty that I have currently a few times.

Prior to the cats I have now, I had a cat who suffered for years with an autoimmune skin disorder (pimphagus--not sure of the spelling) and later developed a rapidly growing cancerous tumor in his leg. I had to have him put to sleep. At the same time, I had a cat that had to be treated for asthma for years, who later had kidney failure. She also developed a growth (black bump--not cancerous) on her back which was surgically removed and biopsied, and she died exactly a week later. She had been walking funny (swaying, disoriented) a couple of days before her death. The vet told me it was vestibular disease and sent me home with some prednisone. I found her dead 24 hours later. Her death occurred in 2007, and the cat previously mentioned died in 2009--a few months before I got my now five year old kitty.

I have never really thought about my house being dangerous, but all of these illnesses make me wonder. I have a chronic cough, by the way. Do you think I just have bad luck with cats, or is something going on here?

It could be incredibly bad luck, it does happen.  What types of cleaners do you use and do you use essential oils in any way.  Kitties are pretty notorious for reacting badly to essential oils.  The prevalence of auto-immune issues and allergies could point to something environmental.  Those issues are also becoming common among kitties though.

I am very paranoid about cleaners and try to only use natural ones without chemicals. Even then, I keep the cats out of the area until everything is totally dry. I did at one time use a cleaner called Whip It (I think) which I was told (and it says it on the bottle) was safe to use around pets. It does have eucalyptus oil in it, but I don't think that I used it around my previous cats--just the ones I have currently. I use a bug spray called Eco Smart, which is also supposedly safe to use around pets and has Peppermint oil, as well as some other oils in it. I can't remember.

You sound like a very conscientious cat mom, so it makes me believe it's an unfortunate coincidence.

Unless of course you had lived someplace else and didn't have a cough, and cat didn't have an autoimmune disorder, but after a short time of living there these things came about.

Then I would highly suspect black mold, possibly inside the walls.

All it takes is for at some point the roof leaked somewhere or another, and moisture was able to come in and the drywall was not replaced.

This is NOT uncommon, and can wreak all sorts of havoc on health, in humans so animals as well.