Originally Posted by SeeDRankLou
Now, normally when one takes medicine, one already has the systems of the illness. You take the medicine, the symptoms continue until the medicine kicks in, if it even works. However, if you have taken the medicine before hand, i.e. a vaccine or preventative medicine, then your body is ready for the illness before the illness is even there, and it can ward it off without symptoms. So, this is why I believe that Zack was unaffected by the JENOVA cells. He was infused with mako energy to make him a SOLDIER, so in essence he had taken the vaccine for JENOVA cells. It's not said that Cloud is treated with both JENOVA cells and mako (Hojo's notes only say JENOVA cells), but he probably was. So Cloud was given the medicine either shortly beforehand or afterwards, apparently not long enough to properly fend off the JENOVA cells. And who knows if it was even enough to do so in the first place. But what saved Cloud from the same fate as the rest of the Sephiroth clones was Tifa. When he saw Tifa, what was left of his consciousness triggered a reaction with the JENOVA cells to make him into the person that he became. Otherwise, he would have just stayed the way he was and one day wondered off to the north crater like the rest of them. When he falls into the lifestream the first time and receives mako poisoning, he kind of like he was heavily injected with the medicine for JENOVA cells. So when they finally finished working and Tifa helped him find his consciousness again, he was himself again. Notice that from here on through the game he no longer experiences any voices or meetings with himself or headaches of any kind. This is just they way I see things though.