Yes. They’re contagious (easily spread by physical contact) from the first tingle you feel on your lips until they are 100% healed. The virus is passed on by direct contact, not in the air.
Cold sores are at their most contagious when the blisters burst, so you should be extra careful to avoid close contact with people until the sores are completely healed and all signs have disappeared.
Since it’s so easy to pass on the virus, when you have a cold sore you need to be particularly careful around those with weak or weakened immune systems, such as newborn babies, people with HIV, children with atopic eczema, and people having treatments such as chemotherapy.
Most things you can do to prevent the spread of cold sores are simple and common sense:
Q. For how long are cold sores contagious?
A. For between 10 days and two weeks – over the full lifespan of the outbreak, from the first tingle experienced by the sufferer to all trace fading away
Q. Are cold sores contagious before I feel tingling?
A. No. You can carry the cold sore virus without being able to pass it on to others. Indeed, most people who carry the HSV-1 virus never develop cold sores, and never become contagious
Q. Are cold sores contagious to babies?
A. Yes. Newborns' immune systems aren’t developed enough to deal with the HSV-1 virus, so you should, where possible, avoid contact with them if you have cold sores. If contact is unavoidable, e.g. because you are the baby's mother, avoid kissing and keep your hands meticulously clean to avoid spreading the virus.
Q. Should I worry about cold sores during pregnancy?
A. It's unlikely there will be any negative effects on your baby, but if you develop cold sores during pregnancy, you should see your GP