Life before the Elves arrived?

One night, while talking to Multilex about the history of Drows, I couldn't help but notice that in between 300 AT and 400 AT, there was an unexplained decline in both Drow and Elven populations. While earlier entries may suggest that Drow diseases caused this, the diseases would only affect Elves, not Drows. This fact got me thinking, and we came to the conclusion that some sort of disease may have been released during the 100 year timespan. This disease would have caused the decline in Drow and Elven populations, and to some it may have even affected their mental capacity; hence, Yorn Dios finding Drow women uninteresting. We are unsure on what caused the disease to be suddenly released, but it was sealed up somehow, in a cavern perhaps. The reason some survived may be that a part of it was still released, but significantly weakened, when the elves arrived, and to them it may have simply been something similar to a common cold. A similar effect may have been on the Drows, and for some this was enough to form a full immunity to the stronger disease.
Then we began to wonder: How was the disease sealed up? Obviously the elves didn't do it, as there are no records of mass sickness until after the Drows arrived, and obviously they didn't have any diseases that would kill them. So we came to the conclusion that some intelligent race, BEFORE THE ELVES, sealed it up. We have plenty of evidence of this;

1. The disease was not brought by Drows, as they where not killed until shortly after they gained total control.
2. It wasn't brought by the Elves, as they had little or none sickness with them before the Drows arrived. Also, there are no listings of mass deaths due to sickness.
3. If the disease was sealed up, the place it was sealed in had to have been totally airtight: and we all know a stupid deer couldn't have the mental power to do this.

This is an amazing discovery, as it may have been a race we have yet to encounter. Unfortunatly, as there where no signs of other races on Korth when the Elves appeared, it is suspected that the race was wiped out by this disease, and we may never have the chance to meet this race in person. But either way, if we dig far enough we may even find remnants or even entire cities of this ancient civilization.