You're welcome!!!

A very good explanation Dragon Mage, but the only thing that i think is wrong, is about the Geostigma part. Before FFVII, Jenova never spread it's cells in the Lifestream and never created Geostigma. This only happens in the end of FFVII. If Jenova did this, then why Sephiroth didn't use the corrupted Lifestream during FFVII? Because it didn't exist yet.
The virus that Jenova created, was just a physical one, and didn't have anything to do with the Lifestream.
But your explanation is very good.
Ah, well, I didn't mean literally Geostigma, but it functioned in basically the same way. This was done to effectively infect everyone and thus to kill them. It wasn't Geostigma per se, but it was a virus spread thoughout the populace to kill them via means of spreading its cells through the lifestream. Geostigma was effectively doing the same thing, however there was no mind behind the virus to purposley infect people. That's why some people were being infected and others weren't. Sephiroth never used the lifestream in such a manner because he would have to re-corrupt it. You see, Jenova, after wiping out the humans, 'recalled' its cells, or basically it just regrouped back together again. Thus, the lifestream was uncorrupted.
And true, it may not have anything to do with the lifestream, but it would be the quickest and most effective method, whereas a physical virus would have a chance to be killed by the hosts immune system and possibly magical means. Why take the chance?

(thanks The Crystal!)