There are many things that can trigger a cold sore. . Many of these factors seem to correlate with time periods when a person's immune system would be expected to be weakened or stressed.
Emotional upset and stress.
Physical stress and fatigue.
Illnesses (including a cold or the flu).
Injury to the lips or skin, such as physical trauma or severe chapping.
Injury to the lips from excessive exposure to bright sunlight or ultraviolet lamps.
Menstruation or pregnancy.
An immune system deficiency.
To help avoid these it is suggested that a person get plenty of rest, drink lots of water, and avoid stress as much as you can.
The last one seems to be the one which might be difficult to control.
Emotional upset and stress.
Physical stress and fatigue.
Illnesses (including a cold or the flu).
Injury to the lips or skin, such as physical trauma or severe chapping.
Injury to the lips from excessive exposure to bright sunlight or ultraviolet lamps.
Menstruation or pregnancy.
An immune system deficiency.
To help avoid these it is suggested that a person get plenty of rest, drink lots of water, and avoid stress as much as you can.
The last one seems to be the one which might be difficult to control.