Causes
Cold sores are caused by the highly contagious herpes simplex virus, which is spread via direct contact with the saliva of a cold sore sufferer, which contains the virus.[2,3] Most people pick it up from their family during early childhood - for example, through kissing someone with an active cold sore.[1] The risk of transmission is highest during the first one to four days of symptoms.[2]

Common triggers
There are a number of factors that can lead to the outbreak of a cold sore, including:[1-5]

1. Illnesses - Such as viruses (particularly colds) or bacterial infections, which weaken the immune system
2. Emotional stress - Recognised as one of the major cold sore triggers, due to its effect on the immune system
3. Strong sunlight and cold winds - Both can damage the thin skin covering the lips, triggering an attack
4. Fatigue and tiredness - Can also lead to cold sores, by weakening the immune system
5. Damage to the lips or surrounding areas - Can result in swelling and inflammation, aggravating the local immune system
6. Menstrual periods and pregnancy - Both have been associated with cold sore attacks.