Aschen

The Aschen are a humanoid race who use biological weapons to defeat their enemies, often by stealth.

In an alternate timeline, SG-1 encountered the Aschen on their homeworld. This meeting led to a treaty whereby the Aschen would defeat the Goa'uld. An undisclosed portion of that same treaty called for the birth rate of Earth to be lowered by 30%.

By 2010, the former members of SG-1 discover that various "vaccines" given to the people of Earth by the Aschen have actually caused the birth rate to drop by 90%, meaning the human race will be extinct within a century or so. And the Aschen will be free to colonize Earth.

However, using the Stargate in conjunction with solar flare activity, SG-1 is able to send a message back in time to 2001, just prior to their original meeting with the Aschen. The mission to their planet is cancelled.

While SG-1 does eventually meet the Aschen through a third party, the warning from the future, while not detailed, makes them suspicious of the Aschen's motives. They quickly discover how the Aschen used "vaccines" to sterilize another planet's population and how they plan to do the same to Earth if given the chance.

SG-1 is able to escape the Aschen's custody and warn Earth. Contact with the Aschen is discontinued.