While cold sores are most contagious when they rupture and ooze fluid, they are actually contagious from the very first symptom, which is normally tingling or itching, until the sore is completely healed, according to EverydayHealth. In most cases, cold sores last approximately seven to 10 days, advises WebMD.
Scabbed over or crusted sores are not completely healed and are, therefore, still contagious, warns Everyday Health. Although cold sores are very contagious, sores do not develop right away after contamination. The virus can lie dormant in the body, and it can take up to 20 days for a sore to emerge, says WebMD.