Cold sores are caused by herpes simplex virus type 1 (HSV-1), a virus carried by roughly 3.8 billion people under age 50, or about 64% of the global population. The virus infects you once, then stays in your body permanently, reactivating periodically in response to specific triggers. Understanding both the initial infection and the reactivation process explains why cold sores keep coming back.
How HSV-1 Infects and Hides
HSV-1 is a remarkably efficient virus. Its outer shell compresses DNA under enormous internal pressure, tens of atmospheres, which allows it to forcefully inject its genetic material into your cells on contact. Once the virus enters the skin cells around your mouth, it replicates rapidly and causes the familiar blistering sore. But that initial outbreak is only the beginning of a lifelong relationship with the virus.
After the first infection, HSV-1 travels along nerve fibers in a process called retrograde axonal transport. It moves away from the skin surface and deep into a bundle of nerve cells called the trigeminal ganglion, located near the base of your skull. There, the virus essentially shuts itself down. It stores its genetic material in a quiet, circular form inside the nerve cell’s nucleus, producing almost no viral proteins. In this dormant state, the virus is invisible to your immune system and can’t be eliminated by antiviral medications. It stays there for life.
What Triggers a Cold Sore to Reactivate
A cold sore outbreak happens when something disrupts the balance between the dormant virus and the immune cells keeping it in check. When the virus “wakes up,” it travels back along the same nerve fibers toward the skin surface and begins replicating again, producing a new sore. The most well-established triggers include:
- Sun exposure and UV light. Ultraviolet radiation damages skin cells, which release an immune signaling molecule called interleukin-1 beta. Researchers at the University of Virginia found that this molecule increases the excitability of the neurons where HSV-1 is hiding, setting the stage for reactivation. This is why cold sores often appear after a day at the beach or a sunburn on the lips.
- Stress. Prolonged stress raises levels of cortisol and other stress hormones in your body. These hormones activate a specific protein pathway inside neurons (the JNK pathway) that essentially gives the virus the green light to leave the nerve cell. When researchers blocked this pathway in lab studies, the virus could no longer reactivate.
- Illness and fever. A cold, flu, or other infection temporarily diverts your immune system’s resources, weakening the local surveillance that keeps the virus dormant. This is why cold sores are sometimes called “fever blisters.”
- Hormonal changes. Fluctuations during menstrual cycles, pregnancy, or other hormonal shifts can trigger outbreaks in some people.
- Cold wind and extreme weather. Physical stress to the lip tissue from cold, dry air or wind can provoke reactivation, similar to the way UV damage does.
- A weakened immune system. Anything that suppresses your immune function, whether from medication, a chronic condition, or fatigue, makes outbreaks more likely.
The same inflammatory signaling molecule, interleukin-1 beta, appears to play a role across several of these triggers. Whether the source is sunburn, illness, or prolonged stress, the end result is the same: neurons become more excitable, and the virus begins its journey back to the skin.
How Cold Sores Spread
HSV-1 spreads through direct contact with an infected person’s skin, saliva, or an active sore. Most people contract the virus in childhood through casual contact like a kiss from a family member. You don’t need to see a visible sore to catch or transmit the virus. The skin can release viral particles, a process called shedding, even when no sore is present and the person feels completely fine. This asymptomatic shedding is one of the main reasons HSV-1 is so widespread.
Contact with saliva, the skin around someone’s mouth, or a visible blister can all transmit the virus. Because shedding can happen on skin not covered by any barrier, there’s no way to fully prevent transmission through physical measures alone.
What a Cold Sore Looks and Feels Like
Cold sores progress through five distinct stages and typically last 7 to 12 days from the first symptom to full healing.
The first stage is a tingling, burning, or itching sensation on or near the lip, usually lasting several hours to two days. No sore is visible yet, but the virus is already replicating beneath the surface. Within about 48 hours, the second stage begins: a cluster of small, fluid-filled blisters forms at the site. During stage three, those blisters break open, leaving a raw, weeping area that is often the most painful phase, lasting about three days. The fourth stage brings scabbing, which lasts two to three days and may involve cracking, bleeding, and itching as the tissue heals underneath. Finally, the scab falls off and the skin returns to normal.
The sore is most contagious during stages two and three, when fluid from the blisters contains high concentrations of the virus. But shedding can occur at any point in the process.
Cold Sores vs. Canker Sores
Many people confuse cold sores with canker sores, but they are completely different conditions. The simplest way to tell them apart is location. Cold sores appear on the outside of the mouth, typically along the border of the lips. Canker sores occur inside the mouth, on the soft tissue of the cheeks, gums, or tongue.
Their appearance differs too. A cold sore is a cluster of small, fluid-filled blisters grouped together. A canker sore is usually a single round sore, white or yellow in the center with a red border. Canker sores are not caused by a virus and are not contagious. Cold sores are both.
Why Some People Get Outbreaks and Others Don’t
Most people infected with HSV-1 never develop noticeable cold sores, or they get one outbreak and never have another. Others deal with recurring sores several times a year. The difference comes down to a combination of genetics, immune function, and trigger exposure. Your immune system maintains a population of specialized cells in the trigeminal ganglion that actively suppress viral reactivation. How effectively those cells do their job varies from person to person. People with stronger local immune responses in those nerve clusters may carry the virus for decades without a single visible outbreak, while others find that any bout of stress or sun exposure sends the virus back to the surface.
Frequency of outbreaks also tends to decrease over time. The body builds a stronger immune memory against the virus with each reactivation, so people who had frequent cold sores in their teens and twenties often find they become less common with age.