How can the infidels (us) "not know until it is too late", yet something will happen "while the world watches in horror". "Not know" could simply mean, we do not know where the attack is coming from "until it is too late" to stop it. I am not sure the first phrase implies anything other than what it says. Of course, a bio attack could have already occurred and no one would know yet...
That is how I interpreted it also. Maybe, he is saying that we will not be able to stop it. We will not have any foreknowledge of the specific attack.