My white blood cell count is extremely low. To the point where, when other doctors (who don't know me) look at my chart, they ask my hematologist how long I have to live. I show no other symptoms of anything that could be tied to a low WBC count. In fact, aside from that blip on the medical record, I'm extremely healthy. My low WBC count should mean that I'm getting infections and the like all the time, but I've missed maybe 10 days of work due to illness in about as many years. I never get sick.
They have been tracking my WBC count monthly for over a year now; and tri-monthly for about a year prior to that. The first instance of low WBC they found was about 5 years ago, although at that time it wasn't freakishly low. It fluctuates, but never seems to go above a certain threshold. I'm now on a six month cycle for checking the count. They have done every test known to medical science on my blood. All of my blood cells and blood components are normal in their morphology and "behavoir" (or whatever), except that my body doesn't make a lot of white ones. Specifically, my neutrophil count is the low one. The doctor explained what it meant to me but I forgot. I dunno. I feel fine.
The doctor kept telling me to call him immediately if I ever got a cold or anything, so I could come in and get a count taken, to see how (or if) my immune system responds to infection, but I haven't been sick in the last 2-3 years. So that's just one they'll never figure out, I guess.
Finally I opted for a bone marrow biopsy (it's not a fun procedure, and it feels gross, but I did get to see a chunk of my own bone without experiencing trauma, so that was kinda cool). My hematologist was thinking that maybe my bone marrow is producing enough white blood cells, but that they were dying off before entering my bloodstream, something called "maturational arrest."
Turns out, my bone marrow is functioning perfectly normally.
So the inexplicability of it is that I should not be as well as I am, and have the blood counts that I do. According to the numbers, I should be down with every possible infection out there, but I'm not.
For the record, when I went in for my bone marrow biopsy, I listened to my iPod as a distraction. The song? Freezepop's "Here Comes A Special Boy" on repeat.

As to the other question...
I have another best friend whom I have met in person. I've known her for over ten years and she lives in my city and I'm godmother to her daughter, etc. So I do have a "normal" best friendship as well. My "internet best friend," well, I can't really say how or why that happened the way it did. It just did. We're close. We connect. Like any friendship, we orbit one another and sometimes that orbit is on an "inswing" and other times it's on an "outswing," but the longest we've gone without talking to one another has been, I think two weeks. So there has been a lot of ground covered via conversation in the past 4 years when you consider that it's been a 2-6 hour long conversation almost daily for 4 years. Even on a conservative estimate (typing in chat takes longer and so therefore a 4-hour chat is about the same as maybe a 1-hour phone conversation, we haven't talked every single day--although it's been close, etc), we've still put a lot of time and effort into this friendship.
This is the person I think to talk to when I need a reality check, when the merde hits the fan; the person I worry about when I know something big is coming up; I guess we have chosen to make one another a priority and that's where the friendship has gained so much strength.
The reason we haven't visited in person truly has everything to do with life circumstance and none of the other dramatic ideas you've proposed (although I agree, those would make for a better story). I am unbelievably broke right now, basically living check to check, so a plane ticket is not an option for me. My best friend is at a place in life where travel is not practical. Although we very badly want to plan a visit, and have been talking about it for a few years now, there is also a sense that things are fine as they are, so there's no need to rush a visit necessarily. A visit doesn't seem, to us, to have the potential to change much, aside from our own sense of curiosity about being in one another's physical presence. These things are important, surely--a virtual hug can never ever be the same as a real one, no matter the feeling between friends--but we both know the visit will happen eventually, and in the meantime we'll keep working on and enjoying what we have.
Sorry it's not more dramatic than that. *shrug* Just two people who really click, I guess.