It is all about the diffrence between what you put into the box of "must happen" and the one with "could happen".
I personally have found out that movies are most enjoyable when you don't start some inane speculation about chances. If it doesn't happen in the movie, it just doesn't happen. Even if you are convinced it's really really really likely, as soon as there is a chance it doesn't, the movie still works.
You think that they would all get the flu, but they don't. That kinda ends it.
People not getting the flu does not obviousely break any laws of physics or violate time and space. Could be worse.