I've been tired since last Friday. And the fatigue must've weakened my immunity -- my upper lip is getting attacked by cold sores. Because I've got nothing to do here at work except for just mindlessly waiting for this program installation to finish, I decided to study about cold sores. (Damn, they hurt a lot.)
Herpes simplex virus type 1 (HSV1). It's the virus that causes the notorious cold sores, or fever blisters. Infection is very common among people -- estimated 9 out of 10 people in the states are believed to have this virus.
How come so many people have it yet there's so little buzz about the disease? The answer is that 1) HSV1 only bursts out when your body is tired and your immunosystem is weakened, and 2) once you get infected by HSV1, you can't get rid of them -- when your immunosystem once again becomes strong and corners those viruses, they retreat and hole themselves up into the central nerve system, doing nothing but biding their time.
The interesting thing is, although the viruses are holed up in the central nerve system, they rarely propagate to other nerve branches. As if they remembered where they lived. That's the reason you get cold sores always at the same spot (for me, it's a bit to the left from the center of the upper lip) over and over.
Oh, and the cousin of HSV1 -- herpes simplex virus type 2 (HSV2) -- is much more devastating; it's the very virus that causes genital herpes. Like HSV1, HSV2 can't be expelled out of your body once you get infected. Once you get a genital herpes, you get to live with it for the rest of your life. And you'll be sorry for your prospective sexual partners for the rest of your life -- genital herpes is, just like cold sores, highly contagious. Scary indeed.
Edit: No matter how sleepy you are, you must not rub your eyes if you touched any of those cold sores but haven't washed your hands afterwards. HSVs can infect your cornea and inflict a serious and permanent damage to your eyes. That's indeed what's keeping me awake now. :D