I have an Itty Bitty, too -and here's her story
Your Itty Bitty is a beautiful cat - what a good person you are for rescueing her and nursing her back to health.
I also have an Itty Bitty who I acquired under similar circumstances. She was dumped at the shelter with her one remaining kitten by her owners who hadn't bothered to get her spayed. I was interested in her and the kitten, but took home two kittens from another litter instead. I told the shelter that if she didn't adopted with her kitten to call me - maybe I could help find her a home.
A few days later, they called - said that she'd developed a URI that wasn't responding to meds and that the vets recommended that she be declared "unadoptable" and destroyed. But since my name was in her file, did I want her? I wasn't expecting a call like that, so I said I'd have think about it and call back the next morning. I couldn't let this cat die because I'd said no, so I went and got her the next day.
I understand exactly why she was so sick: she probably hadn't been fed properly during her pregnancy and lactation (she was so thin!, poor girl), she'd been dumped at the shelter, which is always stressful, her last kitten had been adopted, and she had the additional stress of recovering from a spay surgery. She just gave up, and wasn't fighting off the virus.
But all she needed was some more intensive medical care and some love. She couldn't eat because she was so congested, and I had to start syringe-feeding her. Even though she was obviously feeling so poorly, she loved being held and would curl up against my chest. She wasn't improving, though, so after a few days I took her to my vet for stronger meds and some A/D food to put the weight back on her.
She recovered pretty quickly after that, and is now a happy part of my multi-cat household. She still has some mild chronic URI, but regular doses of L-lysine keep it under control. She plays with the other young cats, and she still loves to curl up on my chest. When I hold her and say "give me a kiss, Itty Bitty," she nuzzles me. Then I say "kiss me again," and she does.
Oh, and she's not so Itty Bitty anymore! She's still a small-boned cat, but she's filled out and looks like a little nutmeg. :)