What does "prone to any sinus infections" mean? If you have an upper respiratory infection, that makes you "prone" to secondary infection, right?

Just because you haven't had a particular illness before doesn't mean you'll never get it. You mention the post-nasal drip, that's literally what precedes a sinus infection!

I mean, sure the Magical, Mystical Coof has powers no man can truly comprehend, such as it's ability to avoid you when you're sitting down at a restaurant or screaming in a cop's face at a riot, but in this case I gotta say this is worth a second thought.