Cold sores are caused by the Herpes simplex virus.  They flare up when you stress, for ladies, it can flare up when your hormones are out of whack or during menstruations.  It's a virus, so you can't kill it.  Once you've got it, it lays dormant in your system and certain events will trigger it to show up.

Best thing to do, is get a medicinal cream or salve to apply to the infected area.  Certain foods can trigger it.  Try reduce your stress levels as best as you can.  Hydrate, eat clean, reduce coffee and alcohol intake.

Don't share cups, forks and avoid intimate contact when the infected area may come into contact with someone else.