So...I have a theory that sort of extrapolates on the plant theory. I do think that the plants are what is releasing the toxins (or whatever) because it starts in parks, etc (and now because of his original title as well). I didn't really debate this part of it...what I focussed on was the trigger: Why did some people get affected at certain times and why didn't others? I think it has to do with mood/emotion. I think that the plants have had enough of us peeps destorying the planet so they decided to kick some ass of their own. The trigger? Negative emotion. If someone starts eminating negative feelings (which theoretically plants respond to), these spores or toxins or whatever get set off as a defence. In other words, mother nature being proactive in nipping a potentially dangerous person in the butt.

1. One of the two girls hears a scream in the park (while the other one reads a book about a killer). Violent crime being committed?

2. The introduction of the mood ring Whalberg carries with him.

3. Mark Whalberg's character is so passive aggressive. The underacting gets this across really nicely and if you take it as that then its actually a pretty sweet performance. He never loses his temper...always monotone. = safe

4. His wife talks about never showing her feelings. She shares this personality trait with the little girl. She goes so far as to say to the little girl "you're just like me, I keep my feelings on the inside as well". = both safe

5. The old lady in the house is unaffected until she freaks out on Whalberg and gets all bent out of shape. = head smashy smashy.

6. When the group is walking through the field with the army dude. Two of the guys at the front of the line start to get in an argument...2 seconds later..."my firearm is my friend."

7. In France, another scream sets off that park's plants.

So, when all is said and done, I think Mr. Shamalan is taking a page from Peter Pan: Think happy thoughts (or at least stay the hell away from negative people if ticked off plants are in the vicinity)
totally! and someone else pointed out to me the use of the color yellow in the film.