It is HARRRRD to contaminate beer.
Remember that label on hand sanatizer that says it kills 99.99% of all germs and human pathogens? Guess what's the active ingredient in most hand sanatizers? Ethanol. Guess what yeast pump into your beer? Ethanol.
A 1.6% ABV solution will kill just about anything that can ruin your beer besides wild yeasts and some bacteria like lacto and brett that are perfectly fine to drink, even though they won't taste very good unless you control them.
Here's a short list of things that a 2% ABV beer would kill pretty easily. You could literally toss these in the wort (at some reasonable volume), and the beer would turn out just fine:
Flesh Eating Virus
Hepatitis A through Z
Notice that "cap from your jug" could easily sneak onto that list.
In fact, saison/farmhouse beer in France was basically developed so that people could drink water that was contaminated with all sorts of nastiness, including black plague. They figured out that making about a 2% beer would make it potable. Beer (or alcohol at least), literally saved humankind throughout history.
Interestingly, the one thing that will jack you up that alcohol DOESN'T kill is ringworm (which is a fungus). That cap wasn't from your jug of ringworm, was it?
Wait, did you say part of your smack pack?? Oh, you're F'd!!