It's been a while, but from what I recall:
What was the aliens' intentions? Did they want to just live peacefully as humans or did they have some master plan in mind?
Their planet was dying so they were looking for a new place to live. Dont remember how they first got here, but they were waiting for our technology to get advanced enough so they could build this teleport thing and bring their planet and people here. Then they tried to release this new virus/disease that would pretty much destroy the human population to make room for all their newly arrived people