Wow. I've never heard of such a thing. It is possible for a contamination to get into an open container and spoil the contents--but 40% alcohol (80 proof) is usually way too high for microbes to survive in. Things happen, I guess. It's too bad it happened to a quality whiskey like Bushmill's, though!
Typically, whiskey does NOT go bad. It doesn't age in the bottle, but will sit there, more or less unchanged, for an indefinite length of time. I have bottles of stuff over ten years old in my liquor cabinet that are still drinkable, but they're not the types of liquor I usually drink.