Often, it's an infected sweat gland.

I find it interesting that you guys were so grossed out by that. Others I know were as well. Things like that don't really affect me that way for some reason.

I once stopped at a motorcycle accident on my way home from work. I was also riding my bike. I picked the wrecked bike up and moved it to a safer spot, then went to assist the rider (who I didn't realize was still on the ground). About the same time I got to him, the paramedics pulled up so I helped them with whatever they needed. Dude had one leg turned around the wrong way, you could see his guts sticking out of a wound in his abdomen and his fingers looked like VERY fresh ground beef.

I got back on my bike and continued riding after they loaded him into the ambulance. I still ride today.

But then, I often use needlenose pliers to treat my ingrown toenails. Just soak my feet for a few minutes, then start peeling the offending nail edge back, tearing it from the rest of the nail, until I get all the way back to the nail bed. Then I get a good grip on the part of the nail I peeled back (using the pliers still), and give it a good yank straight out the front. Pulls a bit of attached stuff out from wherever the nail starts growing, but doesn't bleed too much. I don't have to do that too often, though.

I also get cysts like that in much more, uh... sensitive areas than shoulders. I usually squeeze them until they burst. It's just easier than waiting for them to rupture on their own. Hurts a lot, though. Mine are caused by stress.