But it goes back to "how sick is sick enough?" What ailment did the gimp guy in S. Korea suffer from that made him "immune." Does "cancer" (or whatever the bald, reedy kid in Israel had) or "oldmanism" (or whatever the old man in Israel had) really smell that different enough than a cold? And why would ANY infection or disease matter to a creature that's already dead and not biologically active?
Objectively speaking was the fast moving zombies being able to detect sick humans any worse in terms of a plot than Khan's magic blood being able to bring Jim Kirk back from the dead and cure sick people?
Both are complete fiction. And in both films we suspend our disbelief for the enjoyment of the film.
Now that I think about it - I liked World War Z even moreso than the latest Trek film. The Trek film was laced with as many if not more plot holes than Z but side-by-side Brad Pitt simply is a better actor than either Pine or Cumberbatch.
I get it, slow zombies are only a threat if everyone is stupid during the initial outbreak and allows it to get out of control. It's a conceit of the genre. If the military went in and curb-stomped the shambling hordes on day one of the outbreak, that wouldn't be much of a zombie story.
Part of the suspension of disbelief problem with The Walking Dead
where the Zombies now do rule the world is as you say, either the US military was very slow to react to the threat or they are incompetent.
Also the other problem is the gun ownership percentages in the US and people's ability to defend themselves against the threat. You'd think that quickly people would learn that a shot in the head would end the problem and at least some parts of the media
would be left to tell the remaining populace how to handle the problem.
World War Z's fast moving Zombies are a far more credible way for the world to be completely overrun.