[Kipperriffic]

I think that "Walt poisoned him" is a flawed assumption.

I. It may be that Gus poisoned Brock.

It is easier for Gus to steal ricin that it is for Walt. It is virtually impossible for Walt to steal it. Walt probably doesn't even know how close Jesse is to the kid - when he showed up at Jesse's house, he wans't really in a state of mind to be observing how fun and happy is the relation between Jesse and Brock. And don't even start witll Huell's pat down. You are looking wayyy to much into it to assume that Cigarette was transferred at Saul's office. (I can imagine Saul laughing at Walt's face at the idea).

Lets stick to the most plausible. Gus wanted to use Jesse to kill Walt. Gus, not Walt, knew about Brock and his family and their closeness to Jesse.

But Gus' plan failed when Walt prooved to outsmart him and turn Jesse against Gus instead! The only way Walt survives is if he can be 100% sure Jessie stays on his side, or Gus dies. Now, he could accomplish both tasks of getting Jesse on his side permanently AND getting Gus killed if he could do something to piss Jesse off at Gus enough to kill him. Jesse shows up at night and we have that amazing scene. I think most people missed this: Its not that Walt poisoned the kid, he is TAKING ADVANTAGE of the whole situation. He had no idea why Jesse was there. But after his crazy laugh, he cooks up this scheme of using Gus's plan against him.

Jesse did the stupid thing of mentioning "Poison" at the hospital to Gus. While walking to the car, Gus is thinking: "He knew about poison?" "why didn't he kill Walt" "Ah! Walt must have used the situation against me" "wait a minute, what else did they talk about last night" "where is Walt anyway" "...something is not right"

And now, Gus outsmarts Walt.


II. Brock poisoned himself

the third theory that both Walt and Gus don't have anything to do with the poisoning is the BEST one and imo the most likely. Both Gus and Walt are very sensitive to information now and very suspisious at activities happening around then. That sense of looming danger has walt taking risks of making a BOMB in his house! And the same sense prevented Gus from getting into the car. Brock - probably someone with easiest access to it - accidentally took out the cigarette. Its the whole feeling of danger in the air that has Jesse, Walt, and Gus overreacting to everything - in some cases its a bad thing (i.e. Jesse overreacting and wanting to kill walt) and in other cases, life saving (gus not getting in the car)