Well, Quarian don't have NO immune system, it's just really really weak.xvbones said:snip
Without any immunity system they would have died out, because sex would have been impossible.
They lost ther immune system because it was not neccessary while being on this ships and they hope to find a world, their homeworld or another planet that isn't too hostile, so they can start addapting to a normal live again, and can start rebuilding their immunity systems.
But i don't love tali either. She seems nice and cute but i wasn't interrested in dating her in Mass Effect 2.