The Worst Potential Genetic Nightmare?

[Jeff Gerke]

I am writing a novel involving genetic engineering. What, in the
opinion of readers of this newsgroup, would be the worst possible
thing that could be engineered with this technology? 

I am not an anti-genetics person, by the way, I'm just looking for a
good "or else" scenario for my novel.

What would it be? Making HIV airborne? Hybrid animals/humans? What
would be the worst you could imagine?

Thanks for your help.

Jeff