Fili lunged forward and grabbed hold of his brother by the cloak to steady him so he wouldn't fall. It wasn't long before the enchantment, or the heavy air caused by the sickness of the trees began to affect the group. 

They began to walk slower, their speech began to slur, they were hallucinating and disoriented, and they began to feel light-headed but they forced themselves to keep going. Bilbo looked down at his feet and was alarmed to see that he appeared to be walking backwards. As he looked behind him he was even more disturbed by seeing an image of himself, shaking its head at him.