How is a cold sore contracted?

The cold sore, also called labial herpes, is caused by the herpes simplex virus type 1 (HSV-1).

This virus is transmitted exclusively from one human being to another, most often by direct contact with saliva (kissing, sharing utensils, close contact). Primary infection generally occurs in childhood, between six months and four years of age, and often goes unnoticed. In some cases, however, it may present as a painful herpetic gingivostomatitis, with high fever and oral lesions.

Once contracted, HSV-1 is never eliminated by the body. After the initial phase, it migrates along sensory nerve fibres and establishes latency in the nerve ganglia. It can then remain silent for months or even years before reactivating. Upon reactivation, the virus descends to the skin or lip mucosa, causing the appearance of the characteristic vesicles grouped in a ‘bouquet’ on a red base.

Several factors facilitate these reactivations. Among the most frequently identified are infection, intense exposure to sunlight or cold, stress, fatigue, local trauma (dental procedures, skin fissures) or a decline in immune defences (illness, corticosteroid or immunosuppressive therapy). Such circumstances appear to disrupt the local immune balance and allow the latent virus to resume activity.

Note : It is important to distinguish HSV-1 from HSV-2, which is responsible for genital herpes. In the case of a cold sore, HSV-1 is indeed the causative agent.

Cold sores are contagious from the very first prodromal signs (tingling, burning sensation...), even before the vesicles become visible.

Transmission primarily relies on direct contact with an infected area or with the saliva of a person carrying herpes simplex virus type 1. The most common route of infection is mouth-to-mouth contact, notably through kissing. The virus can also be transmitted via the sharing of items recently contaminated with saliva (drinking glass, cutlery, lip balm, cigarette), although this route is less frequent since HSV-1 survives only briefly outside the human body. Oro-genital practices constitute another transmission route, accounting for the increasing incidence of genital infections caused by HSV-1 rather than HSV-2 alone.

A key point: transmission can occur even in the absence of visible lesions. This is known as “asymptomatic viral shedding.” The virus is present on the mucosal surface without causing symptoms, thus favouring its silent spread within the population. It is one of the reasons why HSV-1 is so widespread on a global scale.

The period of peak contagiousness corresponds to the vesicular phase, when the blisters contain a clear fluid rich in viral particles. The risk gradually decreases as crusting occurs, but it is only considered negligible once complete healing has been achieved. Note that a cold sore takes about ten days to clear up.

Note : Self-inoculation is possible. For example, touching a lesion and then touching your eyes can cause herpetic keratitis, a painful and potentially serious eye infection, which is why it is important to avoid touching a cold sore.

• FELDMAN S. R. & al. Patterns of disease and treatment of cold sores. Journal of Dermatological Treatment (2013).

• LOOKER K. J. & al. Herpes simplex virus: Global infection prevalence and incidence estimates, 2016. Bulletin of the World Health Organization (2020).

• NIBALI L. & al. Associations between host genetic variants and herpes simplex labialis in the TwinsUK cohort. Archives of Oral Biology (2023).

• Organisation Mondiale de la Santé (OMS). Virus herpès simplex (2025).