What happened to Sanura's clan: they started to fall sick and attack everyone still healthy, and when Sanura and her lover tried to appeal to their gods, they discovered the gods were responsible for the plague... because they wanted to make a movie.
When Toph realizes earth-healing would probably let her bend bone, she immediately decides to forget she ever thought about it.
It's implied the Spirit was afraid of what was beyond the Stargate because it used to be an Ancient lab rat, who fled with its friends to a safe world. Then it discovered the Stargate was reactivated, and two of his current host's friends taken by whoever was responsible.
Zuko getting sicker and sicker the more longer he's deprived of sunlight. Even worse, his current caretakers are worsening the issue by refusing to let him go outside because they fear he could get abducted or catch some local illness. Grade-A Adult Fear, here.