They can either not get any illnesses at all, or they can get one.
I've played SA2 through 100% twice (my save was deleted once :P), and none of my chaos ever got sick as far as I can remember. The doctor always seems to say your chao is fine, even if you'd beat it up and throw it around all the time.
You have to buy it
Sonic Advance 1, 2 and 3 (Gameboy Advance) have chao gardens which are small and not very great, but Sonic Adventure (Dreamcast), Sonic Adventure DX (GameCube), Sonic Adventure 2 (Dreamcast), Sonic Adventure 2 Battle (GameCube) all have chao gardens where you can do the most things and raise your chao the best and most effective. Sonic Adventure 2 Battle is the most convinient and shows you all you need to know about a chao, and it's a really good game to raise them in by using animals (and Chaos Drives) as a means of training them. Sonic Advance 1 and 2 can connect up to Sonic Adventure 2 Battle and Sonic Adventure DX via a Gameboy to GameCube link cable and you can exchange chao with.
When ignored or abused I believe
buy it in the black market
Attack it. Also, it's Sonic Adventure 2 Battle.
Sonic Adenture 2 is the original game for the Dreamcast and Sonic Adventure 2 Battle is a remake for the Gamecube. Sonic Adventure 2 Battle has better graphics, chao karate class, chao activities and chao shops.
Sonic adventure battle 2
Yes there are chaos in sonic adventure 2 battle
Gold chao can only be found in sonic advance for the gameboy advance. Then you put it on sonic adventure battle
Sonic Adventure Battle (DC/GC) and Sonic Adventure Battle 2 (DC/GC)
No
There is no chao island. sorry.
No the chao gets sonic hair.
You Can In Sonic Adventure 2 Battle. I Think you can in DX.
You don't.
Yes.
this is how i did it i traded my chao from sonic dx to another chao from sonic advance 2 for the GBA then i traded that same chao to my one of my chao in sonic 2 battle