Cubby's Mom,

DEFINITELY keep him separated from your other cats till he's checked out by the vet. Also, you need to gradually introduce the new cat to the old residents. If you don't know how, see the link:

http://petoftheday.com/talk/showthre...troducing+cats

Here is some more suggestions on gradually introducing your kitty.

1) keep it separate for a few days..
2) put an item of clothing, bedding or other that has the smell of your other kitties and then place a piece of material that the new kitties has around your regular ones. This give them a preview of who has just joined their home..

3) Feed the new kittie separately for a week or two to prevent aggression from the others.

4) Make sure you give your regular kitties more love and attention than usual and certainly more than the new one..