you could have caught the cold sore virus years ago. it stays dormant in the body until something triggers it - having a cold, being run down etc. i've only just got rid of the 2 i had - i hate them, they are horrible. they start off with a bump on your lip that feels warm and within a couple of hours, its a big scab on your lip!!! when u get one, use your own towel, glass etc cos u dont wanna pass it on and always wash ur hand after u touch it. also dont pick it, i always do and i have little tiny scars on my lips
http://www.zovirax.co.uk/faq.html#catch