Cold sores are caused by the herpes simplex virus (HSV), an infection that attacks the skin and nervous system [source: McKinley]. There are two main strains of HSV, appropriately called HSV-1 and HSV-2. HSV-1 generally causes cold sores and HSV-2 is usually the cause of genital herpes, but each virus can cause both mouth and genital sores [source: Mayo Clinic].
Cold sores occur when HSV is passed from an infected person to an uninfected person. HSV is contained in the liquid that leaks out of a sore or blister when it breaks open. It is spread when an uninfected person comes in contact with this fluid, either by touching it directly or through the saliva of the infected person. HSV can be passed by sharing eating utensils, razors or towels, by kissing an infected person, or by directly touching the infected area. HSV can also be sexually transmitted [source: Mayo Clinic].
Once a person is infected with HSV, cold sores are likely to reappear a few times per year. However, cold sores are often triggered by specific events or conditions. By avoiding these circumstances, a person can minimize his or her cold sore outbreaks. Triggers include overexposure to sunlight, stress, fatigue, food allergies and hormonal changes -- particularly those of women during the menstrual cycle. Injury to the lips or gums -- or dental treatment that puts stress on areas around the mouth -- can also set off an outbreak. People with weak or impaired immune systems tend to have more cold sore outbreaks, and those cases are typically more severe [source: Web MD].
In the next section, discover different ways to get rid of cold sores.