It is very easy to spread a cold sore by kissing. Cold sores are caused by the herpes simplex virus, and a person who has been infected by this virus can spread it to an uninfected person by kissing even when the cold sore is not particularly active. It is also possible to spread a cold sore by kissing when the cold sore has already healed over and disappeared, although this is less likely to occur.
The virus that causes cold sores can infect any part of the human body, but spreads most easily to the areas around the mouth and genitals. Two different types of the herpes simplex virus exist. HSV–1 is more likely to infect the mouth and cause cold sores, while HSV–2 typically infects the genitals, but can also infect the mouth and produce cold sores. The virus can survive outside of the body for short periods of time but is most often transmitted by direct physical contact. Prolonged contact increases the risk of infection.
The herpes virus establishes itself in the nerves that lead away from the surface of the skin. In many cases, the immune system can suppress the virus, and symptoms never occur. Other people infected with the virus will suffer from periodic viral outbreaks, which take the form of cold sores on the mouth and lips. These sores and the tingling that precedes their appearance indicate a high level of viral activity. This elevated level of viral activity means that it is much easier to spread the virus because there are many more viruses on the surface of the skin.
The probability that a person will actually spread a cold sore by kissing is determined by several factors. If the other person involved in the kiss has already been infected with the virus, even in a dormant form, he or she will have antibodies that limit but do not eliminate the chances of spreading the virus to another site of infection. A person with a weakened immune system, on the other hand, is much more likely to be infected. Careful hygiene may reduce the chances of spreading cold sores.
Medication can reduce the risk that someone will spread a cold sore by kissing. Antiviral drugs, primarily acyclovir and penciclovir, are used to treat herpes infections. They reduce the number of viral outbreaks and reduce the severity of outbreaks. By so doing, they make it less likely that a person will spread a cold sore by kissing, but do not eliminate the risk entirely.