How do I know that I have a cold sore?

Location of the herpes outbreak

Cold sores are most frequently located on the lips at the place where the lips meet the skin and the mucosa. This is known as the vermilion border.
These herpes outbreaks can also be found occasionally on the cheek, chin and nose. In individuals who have their immune system suppressed by medications or infections such as HIV/AIDS the herpes outbreaks can also appear in the mouth and can sometimes be seen as chronic superficial ulcers.

Has it occurred in the same place before?

Herpes simplex infection is the most common cause of a rash that recurs in the same place or location on the skin. There is another condition that produces a recurring rash in the same location and this is much less common and is called a Fixed drug eruption. This is an allergic reaction that manifests or shows itself only in a small local outbreak whenever the drug is taken.

There are clues that an outbreak is about to appear.

There are symptoms that some people can recognize that indicates that a cold sore is on its way. These are called prodrome or prodromal symptoms.
Some people will experience a sense of tiredness the day before the recurrence of the cold sore. Others feel a tingling, or numbness, or just a feeling of discomfort at the site of the future outbreak. Some will feel subtle tenderness or a sense of localized fullness or swelling in the skin.
It is important to cold sore sufferers to be aware of these symptoms as they allow early intervention or treatment that could abort or shorten the duration of the outbreak.

There is usually a specific cycle of events in the life cycle of a cold sore. This will usually last for 10-14 days.

1. Skin appears normal but there may be localized redness and swelling which is seen in response to the virus multiplying in the skin cells, this is called inflammation
2. Small blisters appear that often seem to join together within a few hours at the site of redness or inflammation. Some of the blisters may look like small red bumps. Usually this area is now very tender.
3. The blisters break and a moist sore can appear, this is the most painful stage.
4. A crust or scab forms over the sore. It is important to keep the sore/scab clean because bacterial infections can occur under the scab. The scabs usually become dry and eventually fall off.
5. The area may stay slightly red for a couple of weeks as the skin finishes healing.