Cold sores happen when a common virus called herpes simplex virus type 1 (HSV-1) reactivates from a dormant state inside your nerve cells. About 3.8 billion people under age 50 carry this virus globally, roughly 64% of the world’s population. Most picked it up in childhood and may not even know they have it until a sore appears.
How the Virus Gets In
HSV-1 typically enters your body through a break in the skin or through moist tissue like the inside of your mouth. This usually happens through direct contact: kissing someone who carries the virus, sharing utensils, or touching a sore. Children often pick it up from a parent or caregiver. The first infection may cause a noticeable outbreak of sores, or it may produce no symptoms at all.
Once inside the skin cells of your lip or mouth, the virus does something that makes it nearly impossible to eliminate. It finds the free nerve endings embedded in the infected tissue and fuses directly with the nerve cell membrane. From there, it travels backward along the nerve fiber, a process called retrograde axonal transport, heading deep into the nerve toward a cluster of nerve cells near the base of your skull called the trigeminal ganglion.
Where the Virus Hides
The trigeminal ganglion is the virus’s permanent home. This bundle of nerve cells serves as a relay station for sensation in your face, which is why cold sores tend to show up on or near the lips. Once HSV-1 reaches these neurons, it essentially powers down. It stops making the proteins it needs to replicate and instead produces only a single molecule called the latency-associated transcript (LAT). Because the virus isn’t actively building copies of itself, your immune system can’t detect it. There are no viral proteins on the surface of the neuron for immune cells to recognize.
LAT also protects the nerve cell from self-destructing. Your body has a built-in kill switch called apoptosis that destroys infected cells, but LAT blocks this process in the host neuron, keeping both the nerve cell and the virus alive indefinitely. Your immune system does station specialized T cells in the trigeminal ganglion, essentially standing guard. But the virus remains invisible to them as long as it stays dormant. This is why HSV-1 is a lifelong infection. No current treatment can reach or clear the virus from these nerve cells.
What Triggers a Cold Sore
A cold sore appears when something disrupts the balance that keeps the virus dormant. The virus “wakes up,” begins replicating, and travels back down the nerve fiber to the skin surface, where it infects skin cells and produces a visible sore. Several well-established triggers can set this off:
- Sunlight exposure. UV radiation on the lips is one of the most reliable triggers. It damages skin cells in ways that can signal the virus to reactivate.
- Physical or emotional stress. Stress hormones alter immune function and change the chemical environment around nerve cells, which can tip the balance toward reactivation.
- Illness or immune suppression. A fever, a bad cold, or anything that weakens your immune response gives the virus an opening. This is where the name “fever blister” comes from.
- Hormonal changes. Menstruation is a common trigger for some people, likely because of shifts in hormone levels that affect immune activity.
- Physical trauma to the area. Dental work, cosmetic procedures on the lips, or even windburn can trigger an outbreak.
These triggers work through different biological pathways, but they converge on the same result: the virus switches from its silent state to active gene expression, begins producing new viral particles, and sends them back down the nerve to the skin. Not every trigger causes an outbreak every time. Some people get cold sores several times a year, while others who carry the virus never get one.
The Stages of an Outbreak
A cold sore runs its course over one to two weeks in a fairly predictable pattern. On day one, you’ll notice tingling, itching, or numbness on your lip or nearby skin. This is called the prodromal stage, and it means the virus has already reached the skin surface and begun replicating. Within a day or two, small fluid-filled blisters appear, usually clustered together. These blisters are packed with active virus, making this the most contagious phase.
The blisters eventually burst and merge into a shallow open sore, which then crusts over into a scab. The scab typically falls off within six to fourteen days of the start of the outbreak. The skin underneath may look slightly pink or red for a few more days before fully healing. You’re contagious from the moment you feel that first tingle until the sore has completely healed over with new skin.
Spreading Without Symptoms
Cold sores aren’t the only time the virus can spread. HSV-1 can travel to the skin surface and shed in saliva without producing a visible sore or any symptoms you’d notice. Research shows that at least 70% of people who carry HSV-1 shed the virus asymptomatically at least once a month, and many shed it more than six times per month. Each shedding episode is brief and involves a small amount of virus, so the transmission risk from any single episode is low. But it explains why so many people contract HSV-1 from someone who has never had a visible cold sore.
Why Antivirals Work Best Early
Antiviral medications work by blocking the virus’s ability to copy itself. They’re most effective when taken during the prodromal stage, that initial tingling or burning before blisters form. Starting treatment within the first day of symptoms can shorten the outbreak and reduce its severity. If you wait until blisters have already appeared and the virus has fully replicated, antivirals do much less.
This is why people who get frequent cold sores often keep a prescription on hand so they can start at the first sign. For people with very frequent outbreaks, daily antiviral therapy can reduce the number of recurrences significantly. These medications don’t eliminate the dormant virus from the trigeminal ganglion. They only limit the damage during active replication, which is why outbreaks can still return once you stop taking them.
Why Some People Get Them and Others Don’t
The majority of people who carry HSV-1 never get a cold sore, or get one once and never again. A smaller group gets recurrent outbreaks, sometimes several per year. The difference comes down to individual immune response, genetics, and the specific triggers a person encounters. People with robust T cell responses in the trigeminal ganglion tend to keep the virus suppressed more effectively. Those with weaker local immune surveillance at the nerve level experience more frequent reactivation, even if their overall health is fine. Stress, sleep deprivation, and sun exposure stack the odds further by temporarily suppressing the immune activity that keeps the virus in check.