Cold sore: How do you catch oral herpes?

How is a cold sore contracted?

Nearly one in two people carries the virus responsible for cold sores. Yet not everyone develops visible outbreaks. Under what conditions does one contract a cold sore? Let’s review the transmission pathways and triggering factors.

Published on March 5, 2026, updated on March 5, 2026, by Pauline, Chemical Engineer — 5 min of reading

What are the causes of cold sores?

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

This virus is transmitted only from one human to another, most commonly through direct contact with saliva (kissing, sharing utensils, close contact). Primary infection generally occurs in childhood, between 6 months and 4 years of age, and frequently goes unnoticed. In some cases, however, it can manifest 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 fibers and establishes latency in the nerve ganglia. It can then remain dormant for months or years before reactivating. Upon reactivation, the virus travels back to the skin or lip mucosa, causing the appearance of characteristic vesicles arranged in a “cluster” on a red base.

Several factors promote these reactivations. Among the most frequently identified are infection, significant exposure to sunlight or cold, stress, fatigue, local trauma (dental procedures, cracked skin), or a reduction in immune defenses (illness, corticosteroid or immunosuppressant use). These situations appear to disrupt the local immune balance and allow the latent virus to reactivate.

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

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How are cold sores transmitted?

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

Transmission occurs primarily through direct contact with an infected area or through the saliva of an individual carrying herpes simplex virus type 1. The most common form of infection is mouth-to-mouth contact, especially kissing. The virus can also be transmitted via the sharing of items recently contaminated with saliva (glass, utensils, lip balm, cigarette), although this route is less common because HSV-1 survives for only a short time outside the human body. Oral–genital practices represent another transmission route, explaining the increasing occurrence of genital infections due to HSV-1 rather than exclusively HSV-2.

A key point: transmission can occur even in the absence of visible lesions. This is referred to as "asymptomatic viral shedding." The virus is present on the mucosal surface without causing symptoms, which promotes its silent spread in the population. This is one reason why HSV-1 is so widespread globally.

3.8 billions

People under 50 years of age are carriers of HSV-1.

64%

A significant portion of the world population is infected with HSV-1.

The period of maximal contagiousness corresponds to the vesicular phase, when the blisters contain a clear fluid rich in viral particles. The risk gradually decreases as the crust forms, but it is only considered zero once complete healing has occurred. Note that a cold sore takes about ten days to disappear.

Note : Self-inoculation is possible. For example, touching a lesion and then bringing your hand to 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.

Tips for preventing the onset of a cold sore.

Avoid direct contact during high-risk periods : Do not kiss someone with an active cold sore or showing prodromal signs (tingling, burning).
Do not share items that come into contact with the mouth : Glasses, utensils, lip balms, toothbrushes, or towels can become contaminated by saliva.
Wash your hands regularly : In particular, after touching a lesion. This limits the risk of self-contamination to the eyes or other areas of the skin.
Protecting your lips from the sun : UV exposure is a common trigger for HSV-1 reactivation.
Maintain a healthy lifestyle : Intense stress, fatigue, and infections that promote cold sore outbreaks mean that getting sufficient sleep and eating a varied, balanced diet can help limit the risks.