Cold sores are caused by herpes simplex virus type 1 (HSV-1), a virus carried by roughly 64% of people worldwide under age 50. That’s about 3.8 billion people. The virus spreads through contact with sores, saliva, or skin around the mouth, and once you have it, it stays in your body permanently, hiding in nerve cells and reactivating periodically to produce outbreaks.
Before diving deeper, one important clarification: cold sores and canker sores are not the same thing, and the distinction matters if you’re trying to figure out what’s happening inside your mouth.
Cold Sores vs. Canker Sores
Cold sores almost always appear on the outside of the mouth, typically along the border of the lips. They look like clusters of small, fluid-filled blisters. Canker sores, by contrast, form only inside the mouth, on the inner cheeks, lips, or tongue. They appear as single round white or yellow sores with a red border. Canker sores are not caused by a virus and are not contagious.
If you’re seeing sores inside your mouth, you’re more likely dealing with canker sores than cold sores. But HSV-1 can occasionally cause sores on the gums or roof of the mouth, particularly during a first infection. The blister-like appearance and clustering pattern are the giveaway that a virus is involved.
How HSV-1 Infects and Hides
When HSV-1 first enters your body, it replicates in the skin and mucous membranes around the mouth. From there, it travels along nerve fibers into clusters of nerve cells near the base of the skull. Once it reaches these neurons, the virus essentially shuts itself down. It deposits its genetic material inside the nerve cells and goes dormant, producing almost no viral proteins. Your immune system can’t detect it in this state.
This dormancy can last weeks, months, or years. During latency, the virus continuously produces a specific type of genetic transcript while keeping everything else silent. It’s a survival strategy: the virus stays invisible to immune defenses while keeping itself ready to reactivate when conditions change.
What Triggers a Cold Sore Outbreak
Reactivation happens when something disrupts the nerve cells harboring the virus. Researchers have found that when neurons experience a state of hyperexcitation, the virus senses this change and seizes the opportunity to wake up. It then travels back down the nerve fibers to the skin surface, where it begins replicating again and producing visible sores.
The most well-established triggers include:
- Illness or fever, which is why cold sores are sometimes called “fever blisters”
- Sun exposure, particularly UV light on the lips
- Emotional stress
- Menstrual periods
- Physical injury to the area, including dental procedures
- Surgery
Not everyone with HSV-1 gets frequent outbreaks. Some people experience them several times a year, while others have one outbreak and never another. The frequency often decreases over time as your immune system builds stronger responses to the virus.
How Cold Sores Progress
A cold sore outbreak follows a predictable five-stage pattern and typically lasts 7 to 12 days from start to finish.
The first stage is a tingling, burning, or itching sensation at the site where the sore will appear. This prodrome phase lasts several hours to two days and is the most important window for treatment. Within about 48 hours, the area develops into one or more fluid-filled blisters. These blisters then break open around day three or four, leaving a raw, weeping area that is usually the most painful phase. A scab forms over the next two to three days, often cracking and itching as it dries. Finally, the scab falls off and the skin heals completely.
When You’re Contagious
The virus spreads through skin-to-skin contact during both active outbreaks and periods when no sores are visible. This “asymptomatic shedding” is surprisingly common, and it’s actually how most transmission happens. One study found that 70% of HSV transmissions occurred during periods when the infected person had no visible symptoms. Shedding without symptoms is most frequent in the months and years soon after you first acquire the virus, then gradually becomes less common.
You’re most contagious when sores are open and weeping, but the possibility of spreading the virus exists anytime. Kissing, sharing utensils, and sharing lip products are all common routes of oral transmission.
Spreading the Virus to Other Body Parts
HSV-1 can spread from your mouth to other parts of your own body, a process called autoinoculation. The most concerning example is the eyes. Most cases of HSV eye infections start after the virus has already infected the mouth. Touching a cold sore and then rubbing your eyes can transfer the virus, potentially causing a corneal infection that affects vision. Washing your hands thoroughly before touching your eyes during an outbreak is one of the simplest ways to prevent this.
Treatment and Prevention
Antiviral medications are most effective when taken at the very first sign of an outbreak, during that initial tingling stage before blisters form. Starting treatment after blisters have fully developed still helps but shortens the outbreak less dramatically. If you get frequent cold sores, your doctor may recommend keeping a prescription on hand so you can start it immediately when you feel that familiar tingle.
For prevention, the evidence is mixed on many popular remedies. Sunscreen applied to the lips does protect against outbreaks triggered by UV exposure in controlled lab settings, but studies using natural sunlight showed less clear results. Using SPF lip balm before sun exposure is still a reasonable precaution given how consistently UV light triggers reactivation. Lysine supplements, despite their popularity, have not shown evidence of preventing cold sores in clinical trials reviewed by Cochrane, one of the most rigorous bodies evaluating medical evidence.
The most reliable prevention strategies focus on managing known triggers: protecting your lips from sun, managing stress, and maintaining general immune health through sleep and nutrition. For people with frequent outbreaks (six or more per year), daily antiviral medication can significantly reduce how often sores appear.