It's why doctors warn people not to kiss young babies when they have a cold sore after reports of infants dying from a brain infection.
A teenage girl who has never had a boyfriend because of her chronic, disfiguring cold sores says she has found a near-cure with a £7.95 liquorice balm.
Indigo Faulkner, 18, has suffered with them since she was five – and claims they became progressively worse and more frequent.
Around two thirds of the global population are estimated to have HSV1.
The cold sore strain can also cause genital herpes – which is why experts warn against having oral sex when you have an outbreak on your lip.
Miss Faulkner was brought up in Spain and moved back to the UK five years ago when the change caused her cold sores to get worse.