Cold sores or fever blisters not only are ugly and painful, they are contagious! It starts out as a tingle somewhere on or near the lips and grows into an unpleasant bunch of little virus-filled ulcers.
They can be passed along to others by kissing contact, sharing lipstick or ChapStick or even using the same straw or fork.
There are medications to treat the outbreaks, but the best way to prevent them is to avoid excess sun exposure, used SPF/personal ChapStick, and do your best to stay happy and healthy (as stress and poor nutrition can cause outbreaks).
Frequent hand washing is a great way to prevent the spread of cold sores, not to mention a lot of other cooties!