I am a big DC guy, and read/collect most of the titles. Perhaps because of that, and the blur of Final Crisis and everything else, I've forgotten when the meta-gene virus first was released? Now that I'm typing this... was it Countdown?
Depends on if we're talking about the Lex Luthor meta experiements or the first appearance of the metagene and everything involved with it. Millenium first introduced the coolest concept in the DCU, that being 'metahumans'.
I thought that was Invasion, with the Dominators detonating their meta-gene bomb to cause all those with the meta-gene to become sick.
The way I remembered Invasion, the metagene had started to spread so the Dominators, the Khund, and the Thanagarians invaded Earth to keep the metahumans from becoming so powerful that Earth would never be able to be dominated. In short, the Dominators were acting out the Bush Doctrine -- a pre-emptive attack -- fifteen years before anyone had heard of the Bush Doctrine. Then one of the Legion of Super-Heroes Annuals explains how Valor took those with the active metagene in the wake of Invasion and settled planets throughout the galaxy, which leads, eventually, to the United Planets of the 30th century. Mind you, these are twenty year-old memories, so it's possible that I've confused Millennium and Invasion in some respects.
Ok, to be clear.. I'm talking about recent meta-gene virus... Upon further recollection, the virus in Countdown as a "general" virus that wiped out mankind, not a meta-gene virus.