Cold sores are caused by the herpes simplex type 1 virus. You will usually have your first exposure to the virus during childhood. After your first infection, the virus stays in your body until something triggers it. Stress is one of the major triggers of cold sores. When your body's defences are down during times of stress, the virus takes this opportunity to cause an infection.
Some people can often get cold sores after a skiing trip.
People with eczema, or lower immunity caused by certain drugs, can also get cold sores frequently. Some women often get cold sores around the time of their period. A cold sore usually starts out as a tingling or burning feeling. Some people say the tingling feeling is a reminder they will get the painful cold sore soon. The skin where the cold sore will appear can be itchy as well.