Cold sores, also known as fever blisters, are caused by a virus. They usually appear around the mouth and on the lips. They are highly contagious but not dangerous.
About 60% of the population have suffered cold sores at some point in their lives. On average, people who get cold sores have 2 or 3 episodes a year, but this figure can vary significantly from person to person.
The virus that causes cold sores is herpes simplex 1, a cousin of herpes simplex 2, which causes the well-known sexual disease. About 80% of the people in North America have dormant (inactive) herpes 1 virus living permanently in their body.
The virus typically resides in a dormant state within the body's nerve cells. The body's immune system is normally able to keep the virus in its inactive state. When an infected person is exposed to a "trigger," or if the immune system is weakened, then the virus quickly multiplies and spreads down the nerve cell and out onto the skin, usually on the lips. This produces the characteristic tingling sensation and subsequent clusters of blisters.
Specific triggers include:
You can catch the virus if you come into direct contact with the cold sore blisters or the fluid inside them, which contains a high number of the viruses. This can easily happen through touching the hands of someone who has touched their blisters. It can also occur through sharing toothbrushes, cups, cutlery, face cloths, towels, lipstick, or other personal items that have been contaminated with fluid from the blisters. Once the blisters have stopped oozing or have crusted over, the person is no longer contagious.
People who get cold sores may feel some unusual sensations around the lips in the 24 hours before the blisters appear, including tingling, burning, pain, or numbness. This is called a prodrome or warning sign that cold sores will appear at these spots. The skin turns red and blisters form. They ooze a clear liquid for a few days that dries to a yellow crust over a period of about 3 to 5 days. There is usually some pain in the first few days after the cold sores break out, but this often disappears as the cold sore crusts over. Complete healing takes from 10 to 14 days.
The condition typically causes a cluster of lesions or blisters at a site around the lips. Areas other than the lips such as the inside of the mouth, around the nostrils, or even the surface of the eyes, can also be affected. It is possible to spread the virus to other parts of your body if you touch the blisters and then touch yourself elsewhere. Cold sores inside the mouth can be problematic, interfering with talking and eating. If the virus infects the eye, it can damage the surface leading to vision loss. Very rarely, it can get into the brain, causing viral meningitis or encephalitis. The virus that causes cold sores, herpes simplex 1, can also be spread to the genitals during oral sex, leading to genital herpes.
Herpes simplex 1 never goes away completely, so cold sores can return later on if they are triggered again. Most cold sores don't leave scars; however, if an open blister becomes infected with bacteria or the lesions tend to return at the same site, scarring may result. People with weakened immune systems tend to get more cold sores and heal slower.