Cold sores are always contagious, according to Mayo Clinic. Even if you do not have an active cold sore but are infected with the herpes simplex virus that causes cold sores, or HSV-1, you can transmit the virus that causes cold sores and infect someone with whom you have close, personal contact.
Once someone is infected with the herpes virus, he becomes a carrier regardless of whether he gets cold sores, explains Richard J. Whitley for Medical Microbiology. While cold sores are especially contagious once a person feels a tingle or twitch in the spot where a cold sore forms, and even more so once the blister begins to have fluid and pus, this is not the only time a core sore is contagious.
The virus begins to replicate and shed at a much higher rate when a person has an active sore, but it is possible an infected individual sheds the virus even when an active eruption is not taking place or when the person has never had a single cold sore but is still a carrier of the virus, states Whitley. The virus also sheds in the saliva and nose droplets of the carrier and may replicate at any time. About 90 percent of the world population is infected with the herpes virus but not all those people have cold sores, notes Mayo Clinic.
WebMD states that cold sores can appear as late as 20 days after exposure to herpes simplex virus. Cold sore attacks are usually signaled by itching, redness or pain near the exposure site.