Cellulitis. For me the highlight of the description linked above is "left untreated, the spreading bacterial infection may rapidly turn into a life-threatening condition."
I guess now's not the time to get all hippie health food with the saline nasal spray and chamomile compresses. My initial response is always just to let things run their course, but when running its course = death, I have only my action-hero husband to thank for the motivation. ("If that thing was on Jackson's nose you'd have taken him to the doctor three days ago," said he, and it's the truth.)
With me it's a combination of laziness and a morbid curiosity: how bad will it get? What does my face looked like when it's all fucked up? A similar impulse was behind all the self-portraits I took after I had my bike accident. Did I ever tell you about that? I will, as it serves a classic example of yet another thing you shouldn't do without the supervision of professional stunt men.
Oh, let's just take one more look before it goes away:
Now it's off to the drug store for a heroic dose of antibiotics.