Biological kill-switches or something along that line is my guess and the person who is PoTUS has the button, trump refuses to use it but my guess is if hillary was in and WWIII started, she would have flipped it to ensure almost everyone dies spare 1% of the population.
Or from my earlier post from last year, satanic ritual of using baby parts in foods and drinks on every level would bring people to the hospital, just to be 100% certain they didn't have something like kuru or some other mental degenerative disease.