There's that nasty little bugger. A cold sore is caused by an outbreak of the herpes simplex virus 1.

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It's morning, and the mirror is showing you something you'd rather not see -- a cold sore.

You were pretty sure you'd felt that familiar tingle on your upper lip a couple days ago, but hoped that it wasn't the prodrome -- the sensation that heralds the arrival of a cold sore. Here it is now, though, plain as day. In a few more days it will turn yellow and begin to ooze before it becomes crusty and eventually dries and fades like it was never there. You'll be able to return to your normal (and less embarrassing) life.

Try not to feel too badly about it. Cold sores are very common and the virus that causes them is even more so. In the United States, 30 to 90 percent of adults have antibodies present in their bloodstream designed to ward off herpes simplex virus 1 (HSV1), the virus that causes cold sores [source: NLM]. This means that all of these people have been exposed to the virus and their immune systems have taken measures to prevent infection. Some people do have outbreaks (as your mirror told you) and many others go through life never knowing they're carriers.

Active herpes is transferred from person to person contact -- such as kissing a person with an infectious cold sore -- or from person to object, like sharing a razor with a person in the midst of an outbreak. The virus can also be exchanged from one carrier to another without either person ever suffering an outbreak. Once an outbreak has occurred, the virus infects nerve cells in the face, where it remains and can lead to further eruptions [source: Mayo Clinic].

In the nerve cells, the virus essentially hides from an attack of antibodies sent to destroy it. When a human's immune system is suppressed, like during menstruation or periods of stress, the virus replicates itself and travels to the skin cells to cause yet another cold sore.

The HSV1 specifically locates itself in the trigeminal ganglion, a bundle of nerves located by the jaw and temple. However, recent research has turned up something unusual; the virus can also be found in the brains of people suffering from Alzheimer's disease. This has some wondering if the relatively harmless HSV1 may have something to do with this mysterious and fatal brain condition.