Neal knew something weird was going on. A few days before, his lip started tingling and felt a little numb. He didn't pay much attention to it then, but now there was a certain throbbing something on his lip and it wasn't pretty. At first Neal thought it was a zit because it was red and tender, but then it blistered and opened up. Neal had a cold sore.

Maybe you've heard of a fever blister — a cold sore is the same thing. They're pretty common and lots of people get them. So what exactly are cold sores and what causes them?

What's a Cold Sore?

Cold sores, which are small and somewhat painful blisters that usually show up on or around a person's lips, are caused by herpes simplex virus-1 (HSV-1). But they don't just show up on the lips. They can sometimes be inside the mouth, on the face, or even inside or on the nose. These places are the most common, but sores can appear anywhere on the body, including the genital area.

Genital herpes isn't typically caused by HSV-1; it's caused by another type of the herpes simplex virus called herpes simplex virus-2 (HSV-2) and is spread by sexual contact. But even though HSV-1 typically causes sores around the mouth and HSV-2 causes genital sores, these viruses can cause sores in either place.

What Causes a Cold Sore?

HSV-1 is very common — if you have it, chances are you picked it up when you were a kid. Most people who are infected with the herpes simplex virus got it during their preschool years, most likely from close contact with someone who has it or getting kissed by an adult with the virus.

Although a person who has HSV-1 doesn't always have sores, the virus stays in the body and there's no permanent cure.

When someone gets infected with HSV-1, the virus makes its way through the skin and into a group of nerve cells called a ganglion (pronounced: gang-glee-in). The virus moves in here, takes a long snooze, and every now and then decides to wake up and cause a cold sore. But not everyone who gets the herpes simplex virus develops cold sores. In some people, the virus stays dormant (asleep) permanently.

What causes the virus to "wake up" or reactivate? The truth is, no one knows for sure. A person doesn't necessarily have to have a cold to get a cold sore — they can be brought on by other infections, fever, stress, sunlight, cold weather, hormone changes in menstruation or pregnancy, tooth extractions, and certain foods and drugs. In a lot of people, the cause is unpredictable.

Here's how a cold sore develops:

• The herpes simplex virus-1, which has been lying dormant in the body, reactivates or "wakes up."
• The virus travels toward the area where the cold sore decides to show up (like a person's lip) via the nerve endings.
• The area below the skin's surface, where the cold sore is going to appear, starts to tingle, itch, or burn.
• A red bump appears in the area about a day or so after the tingling.
• The bump blisters and turns into a cold sore.
• After a few days, the cold sore dries up and a yellow crust appears in its place.
• The scab-like yellow crust falls off and leaves behind a pinkish area where it once was.
• The redness fades away as the body heals and sends the herpes simplex virus back to "sleep."

How Do Cold Sores Spread?

Cold sores are really contagious. If you have a cold sore, it's very easy to infect another person with HSV-1. The virus spreads through direct contact — through skin contact or contact with oral or genital secretions (like through kissing). Although the virus is most contagious when a sore is present, it can still be passed on even if you can't see a sore. HSV-1 can also be spread by sharing a cup, eating utensils, or lip balm or lipstick with someone who has it.

In addition, if you or your partner gets cold sores on the mouth, the herpes simplex virus-1 can be transmitted during oral sex and cause herpes in the genital area.

Herpes simplex virus-1 also can spread if a person touches the cold sore and then touches a mucous membrane or an area of the skin with a cut on it. Mucous membranes are the moist, protective linings made of tissue that are found in certain areas of your body like your nose, eyes, mouth, and vagina. So it's best to not mess with a cold sore — don't pick, pinch, or squeeze it.

Actually, it's a good idea to not even touch active cold sores. If you do touch an active cold sore, don't touch other parts of your body. Be especially careful about touching your eyes — if it gets into the eyes, HSV-1 can cause a lot of damage. Wash your hands as soon as possible. In fact, if you have a cold sore or you're around someone with a cold sore, try to wash your hands frequently.

If they aren't taken care of properly, cold sores can develop into bacterial skin infections. And they can actually be dangerous for people whose immune systems are weakened (such as infants and people who have cancer or HIV/AIDS) as well as those with eczema. For people with any of these conditions, an infection triggered by a cold sore can actually be life threatening.

How Are Cold Sores Diagnosed and Treated?

Cold sores normally go away on their own within 7 to 10 days. And although no medications can make the infection go away, prescription drugs and creams are available that can shorten the length of the outbreak and make the cold sore less painful.

If you have a cold sore, it's important to see your doctor if:

• you have another health condition that has weakened your immune system
• the sores don't heal by themselves within 7 to 10 days
• you get cold sores frequently
• you have signs of a bacterial infection, such as fever, pus, or spreading redness

To make yourself more comfortable when you have cold sores, you can apply ice or anything cool to the area. You also can take an over-the-counter pain reliever, such as acetaminophen or ibuprofen.

Reviewed by: Steven Dowshen, MD
Date reviewed: October 2010