Epididymitis is an infection inside your balls. The tube relying the testes (the actual ball) to your penis (which is also where sperm matures (the tube, not your penis)) gets infected by bacteria. This means that, given you did not have any recent surgery near the afore-mentioned area, some bacteria actually travelled all the way from the tip of your penis down into your testicles. Which would be the same as walking 200 km just to kick someone in the nuts. Either you really hate the guy, or you're an asshole.
Epididymitis is a clear proof against the existence of God. No, seriously. What kind of fucked up mind would even conceive that idea ? "Hmm, what to do next ? Oh, I know, lets make some bacteria that infect peoples balls !" All due respect, but that does not sound like something that would come up in the bible. Anyone who would consciously allow something like that is far from godly.
If left untreated, or in extreme cases, epididymitis can cause infertility, necrosis of the testes (yes, that means your ball becomes black and dead), and chronic pain (as if you have had your jewels cushed five minutes ago). This conditions can lead to medical amputation of your testes (go explain that to your pals). But even amputation only relieves pain in 50% of cases. And you might experience phantom pain. From your balls.
Now I know this topic is more geared towards the male audience (since in most cases, women do not have testicles). And most women probably think we are pussys for complaining about a little pain in our scrotum. And maybe they're right. But balls are a special thing for men. We derive our value form them. For two billions years, it was the only thing that justified our existence. Being able to contribute to your reproductive success without actually reproducing (by helping out relatives who share our genes) is a new and rare occurence, considering the entire history of life on this planet.
Oh, and why am I talking about all this stuff ? Well, I've got a friend who knows this guy who kinda asked him the question, so I did some research, you know, to help him out...