Cold sores are caused by herpes simplex virus type 1 (HSV-1), a remarkably common infection carried by roughly 3.8 billion people worldwide, or about 64% of the global population under age 50. Most people pick up the virus during childhood through everyday contact like a kiss from a parent or sharing utensils, and it stays in the body permanently. What triggers an actual outbreak, though, involves a separate set of factors.
The Virus Behind Cold Sores
HSV-1 is responsible for the vast majority of oral cold sores. After the initial infection, which sometimes produces no symptoms at all, the virus travels along nerve fibers and settles into a cluster of nerve cells near the jaw called the trigeminal ganglion. There it enters a dormant state, essentially hiding from the immune system in a form that produces almost no viral proteins. This is why the virus is never cleared from the body. Your immune system can’t target what it can’t detect.
The virus converts those nerve cells into a lifelong reservoir. When something disrupts the balance, the virus reactivates, travels back down the nerve fibers to the skin’s surface, and produces a new cold sore. Some people experience outbreaks several times a year. Others carry the virus for decades and never get a single visible sore.
HSV-2, the strain more commonly associated with genital herpes, can occasionally cause oral cold sores too, but this is far less common. HSV-2 affects about 13% of people aged 15 to 49 globally.
What Triggers an Outbreak
Carrying the virus is one thing. Having it wake up and produce a cold sore is another. A range of physical and emotional stressors can push the virus from its dormant state into active replication:
- Stress and fatigue: Psychological stress and physical exhaustion are among the most frequently reported triggers, likely because they suppress immune surveillance of the virus.
- Sunlight and UV exposure: Ultraviolet radiation on the lips is a well-established trigger, which is why cold sores often appear after a day at the beach or on a ski trip.
- Illness and fever: Cold sores got their name for a reason. Fevers, colds, and other infections that tax the immune system commonly precede outbreaks.
- Hormonal changes: Menstruation is a recognized trigger for many women, with outbreaks clustering around the start of a period.
- Physical trauma to the lips or face: Dental procedures, windburn, or even aggressive lip exfoliation can set off an episode.
- Extreme temperatures: Both heat and cold exposure can reactivate the virus.
Not everyone has the same triggers, and some people never identify a clear pattern. Keeping a simple log of what preceded each outbreak can help you spot your personal triggers over time.
How Cold Sores Spread
HSV-1 spreads primarily through direct skin-to-skin contact, most often kissing. It can also spread by sharing items that touch the mouth, like lip balm, razors, towels, or drinking glasses. The virus is most contagious when a visible sore is present and oozing fluid, but that’s not the only time transmission happens.
The virus periodically “sheds” from the skin surface even when no sore is visible. This asymptomatic shedding accounts for roughly 80% of all viral shedding episodes. Research on HSV-2 transmission found that 30% to 76% of transmissions occurred when no lesion wider than 1 to 2 millimeters was present, meaning many people pass the virus along without knowing they’re contagious. While those specific numbers come from genital herpes studies, the same principle of silent shedding applies to oral HSV-1.
The Five Stages of a Cold Sore
A typical cold sore runs its course in 7 to 12 days and moves through five distinct stages:
The first stage is the prodrome, a tingling, itching, or burning sensation on or around the lips that lasts several hours to two days. Many people learn to recognize this feeling, and it’s the window where early treatment is most effective. Within about 48 hours, the second stage arrives: one or more fluid-filled blisters form, often in a small cluster. During stage three, those blisters rupture and leave a raw, weeping area that tends to be the most painful part of the process, lasting roughly three days. Stage four brings scabbing, which lasts two to three days and often involves cracking, minor bleeding, and itching. Finally, the scab falls off and the skin heals completely.
Cold Sores vs. Canker Sores
People often confuse cold sores with canker sores, but they are entirely different conditions. Cold sores appear outside the mouth, typically along the border of the lips, and look like clusters of small fluid-filled blisters. Canker sores show up inside the mouth, on the inner cheeks, lips, or tongue, and appear as single round white or yellow sores with a red border. Canker sores are not caused by a virus and are not contagious.
Who Faces Higher Risk of Complications
For most healthy adults, cold sores are a nuisance that resolves on its own. But certain groups face more serious risks. People with eczema or other conditions that compromise the skin barrier can develop a condition called eczema herpeticum, where HSV-1 spreads across large areas of damaged skin. This ranges from mild in healthy adults to life-threatening in infants and people with weakened immune systems.
People with severe atopic dermatitis, especially those who developed it before age 5 or who also have food allergies and asthma, are at higher risk. Anyone on medications that suppress the immune system, such as those used after organ transplants or for autoimmune conditions, should take cold sore outbreaks more seriously, as the virus can spread beyond its usual boundaries.
Managing and Reducing Outbreaks
Prescription antiviral medications are the most effective treatment for cold sores. When taken at the first sign of tingling, they can shorten an outbreak by one to two days and reduce its severity. For people who get frequent outbreaks, daily suppressive therapy can significantly reduce how often sores appear. Your doctor can determine whether episodic treatment (taking medication only when a sore starts) or daily suppression makes more sense based on how often you get outbreaks.
Over-the-counter options like topical creams containing docosanol can modestly reduce healing time if applied early. Lip balms with SPF protection help prevent UV-triggered outbreaks. Keeping the area clean and dry during an active sore, and avoiding touching or picking at it, reduces the risk of spreading the virus to other parts of your body or to other people.
The Lysine Question
You may have seen recommendations for the amino acid lysine as a natural cold sore preventive. The theory is that lysine competes with another amino acid, arginine, which the virus needs to replicate. In tissue cultures, this mechanism holds up. In actual human studies, the picture is less convincing. A review of the evidence found that lysine supplementation appears ineffective for preventing or treating cold sores at doses below 1 gram per day unless combined with a low-arginine diet. Higher doses may offer some benefit, but the evidence is not strong enough to consider it a reliable strategy on its own.