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I've googled off and on for years, but I can't seem to find it.
It's set a little bit into the future. Most humans have been wiped out by a virus, but there are three (or four?) cities left. Every time a new section of land opens up that can actually be lived on, the cities have a virtual competition (war), to decide who gets it.
I remember that the kids that play the simulations are bred for it (take athletic genes and mix them in), and I remember that one of the kids is deformed, but in the simulation they make him seem normal. The players aren't supposed to ever meet, but one of the players is a hacker, and set everything up.
I think I read it as a teen - maybe 15 years ago? I started reading at a college level really early, so it's hard for me to judge. I don't remember how old it is or anything. I remember I got it from a library at school, and loved it. Even though it will probably ruin my memory, I would love to find it again!