Cold sores come from herpes simplex virus type 1 (HSV-1), a pathogen that infects most people during childhood or early adulthood and then stays in the body permanently. The virus hides in nerve cells in a dormant state and periodically “wakes up,” traveling back to the skin to produce the familiar blistering sores around the lips. What makes this cycle frustrating is that the causes of initial infection and the triggers for flare-ups are different problems, and understanding both matters if you want to reduce outbreaks.
How HSV-1 Gets Into Your Body
The initial infection usually happens through direct contact with someone who is shedding the virus, whether or not they have a visible sore at the time. Kissing is the classic route, but sharing utensils, razors, or lip balm can also do it. Once the virus reaches the thin, moist tissue around your mouth, it begins replicating in the surface cells of the oral mucosa. From there, it enters the endings of nearby sensory nerves and travels up to a cluster of nerve cell bodies called the trigeminal ganglion, located near the base of the skull. That ganglion becomes its permanent home.1PubMed. Herpes Simplex Virus Type 1 Infects Schwann Cells via Exosomal CD81, Exploits the MVB-Exosomal Pathway, and Induces Demyelination
Many first infections happen in childhood and produce either no symptoms at all or mild mouth sores that get mistaken for teething pain or a canker sore. Some people go through a more dramatic first episode with fever, swollen gums, and painful blisters throughout the mouth. Either way, the outcome is the same: the virus establishes a lifelong foothold in your nervous system.
How HSV-1 Hides in Your Nerves
Once inside the trigeminal ganglion, HSV-1 switches into a dormant state known as latency. During latency, the virus stops making copies of itself and goes essentially silent. Almost all of its genes get shut down. The only part of the viral genome that stays active produces a set of molecules called latency-associated transcripts, or LATs. These transcripts are not turned into proteins. Instead, they act as a kind of molecular silencer, helping to keep the virus’s active genes locked away under chemical tags on the surrounding packaging material.2PubMed Central. Transcription of the herpes simplex virus latency-associated transcript promotes the formation of facultative heterochromatin on lytic promoters
LATs also appear to help infected nerve cells survive. Without them, more neurons die during the initial infection, which would reduce the pool of hiding spots the virus can use later. Research in animal models shows that viruses lacking functional LATs reactivate less often, not because they are better at staying dormant, but because there are simply fewer latently infected neurons left to reactivate from.3PubMed Central. Expression of the herpes simplex virus type 1 latency-associated transcripts does not influence latency establishment of virus mutants deficient for neuronal replication LATs also directly suppress lytic gene expression in small populations of neurons, which affects how frequently the virus reactivates and how large the latent reservoir remains.4PLoS Pathogens. The HSV-1 Latency-Associated Transcript Functions to Repress Latent Phase Lytic Gene Expression and Suppress Virus Reactivation from Latently Infected Neurons
The practical upshot: the virus is not just passively sitting there. It is actively maintaining its own dormancy through a molecular program that keeps infected nerve cells alive while keeping viral genes switched off. When that balance tips, reactivation happens.
Your Immune System as Gatekeeper
Latency is not maintained by the virus alone. Your immune system plays an equally important role. Specialized immune cells called CD8+ T cells congregate around latently infected neurons in the trigeminal ganglion and actively suppress reactivation through a combination of approaches. Some of these cells kill infected neurons that start showing signs of viral activity. Others use chemical signals to shut the virus down without destroying the neuron.5PubMed Central. Immunological Control of Herpes Simplex Virus Type 1 Infection: A Non-Thermal Plasma-Based Approach
When this immune surveillance is disrupted, reactivation follows. That explains why cold sores tend to appear when your immune system is weakened or distracted by something else: a bad cold, surgery, chemotherapy, or even just being sleep-deprived for a few days. The virus does not need a strong push to reactivate. It just needs enough of a gap in immune pressure to start replicating before the T cells can clamp it back down.
Triggers That Wake the Virus Up
Reactivation triggers are the thing most cold sore sufferers care about most, because they are the part of the cycle you can actually influence. Not all triggers are equally well studied, but several have solid evidence behind them.
Sunlight and UV Exposure
Ultraviolet radiation is one of the best-documented triggers for cold sore recurrences. UV light appears to act both by suppressing local immune defenses in the skin and by directly stimulating viral reactivation in the nerve ganglion.6PubMed. Sunlight is an important causative factor of recurrent herpes simplex In one study, researchers exposed cold-sore-prone volunteers to UV light with and without sunscreen. After UV exposure without protection, about 71% developed cold sores within a few days. When sunscreen was applied first, no lesions appeared.7PubMed. Prevention of ultraviolet-light-induced herpes labialis by sunscreen
That is a striking result, and it holds under experimental UV conditions. Whether sunscreen is equally protective during natural sun exposure is less clear. A Cochrane review found that while sunscreen significantly prevented cold sores triggered by experimental UV, a separate trial using natural sunlight did not show the same benefit.8PubMed Central. Interventions for prevention of herpes simplex labialis The difference might come down to uneven application or other triggers co-occurring during real outdoor exposure. Still, wearing lip balm with SPF protection on sunny days is a low-cost, low-risk habit that most dermatologists recommend if you get frequent cold sores.
Psychological and Physical Stress
Stress is the trigger that gets mentioned most often by cold sore sufferers, and there is biological plausibility behind it. Stress hormones like epinephrine and cortisol affect the nervous system and the immune system simultaneously. In laboratory experiments using nerve cell cultures, epinephrine significantly increased HSV-1 replication in certain types of neurons.9PubMed Central. Stress Hormones Epinephrine and Corticosterone Selectively Modulate Herpes Simplex Virus 1 (HSV-1) and HSV-2 Productive Infections in Adult Sympathetic, but Not Sensory, Neurons It is worth noting that this effect was seen in sympathetic neurons (the ones tied to your fight-or-flight response) rather than the sensory neurons where HSV-1 typically resides, so the direct relevance to human cold sore recurrences is not fully settled.
Physical stress counts too. Fever from another illness, exhaustion, major hormonal shifts like those during menstruation, and immune suppression from medications can all tip the balance toward reactivation. The common thread is anything that either distracts or weakens the immune surveillance keeping the virus dormant.
Local Tissue Trauma
Physical injury to the lips or face is another recognized trigger. Dental work is a classic example. Case reports document severe cold sore outbreaks occurring within days of dental extractions or procedures involving local anesthesia around the mouth. Whether the trigger is the physical trauma itself, the anesthetic injection, or both is still debated.10PubMed. Severe herpes simplex virus type-I infections after dental procedures Cosmetic procedures like lip filler injections and dermabrasion around the mouth are also associated with outbreaks, which is why practitioners often prescribe a short course of antiviral medication beforehand for patients with a history of cold sores.
Other forms of tissue disruption, such as chapped or windburned lips, tattoo procedures on or near the lip line, and facial surgery, can do the same thing. The mechanism likely involves local nerve irritation that signals the trigeminal ganglion in a way that nudges the virus out of dormancy.
You Can Spread It Without a Sore
One of the most underappreciated facts about HSV-1 is that the virus sheds from the mouth and surrounding skin even when no cold sore is visible. A study that tested healthy adults for HSV-1 shedding found the virus on the oral mucosa on roughly a quarter of days sampled, with most of that shedding happening asymptomatically. Among individual participants, the median shedding rate was about 20% of days, and some people shed virus more than 60% of the time.11PubMed Central. Herpes Simplex Virus Type 1 Shedding in Tears, and Nasal and Oral Mucosa of Healthy Adults
This asymptomatic shedding is a major driver of transmission.12IPS Journal of Public Health. Herpes Simplex Viruses 1 and 2: Virology, Pathogenesis, Epidemiology, and Clinical Management: A Comprehensive Review Most people who pass on the virus do so without knowing they are infectious. It also means that simply avoiding contact when you have a visible sore, while helpful, does not eliminate the risk entirely. The amount of virus shed asymptomatically is lower than during an active outbreak, which likely makes transmission less efficient, but not impossible.
Why Some People Get Outbreaks More Often Than Others
If you have a friend who carries HSV-1 but never gets cold sores while you deal with outbreaks every few months, your genes might be part of the explanation. Researchers identified a gene on chromosome 21 called C21orf91 (also referred to as the “cold sore susceptibility gene” or CSSG-1) whose variants correlate with how often people get cold sores. A family-based genetic study found significant linkage between specific variants in this gene and cold sore frequency.13PubMed Central. C21orf91 Genotypes Correlate With Herpes Simplex Labialis (Cold Sore) Frequency: Description of a Cold Sore Susceptibility Gene
A follow-up study in unrelated individuals confirmed those findings. Certain CSSG-1 haplotypes were associated with protection from frequent and severe cold sores, while other haplotypes tended toward more frequent and more severe episodes.14Hum Genome Var. Cold sore susceptibility gene-1 genotypes affect the expression of herpes labialis in unrelated human subjects The exact function of CSSG-1 is still being worked out, but the gene appears to influence either how well the immune system controls the virus during latency or how readily the virus reactivates.
Genetics is unlikely to be the whole story. Differences in viral load at initial infection, the number of latently infected neurons, the robustness of CD8+ T cell responses, and exposure to triggers all contribute. But the CSSG-1 findings help explain a pattern that frustrates many people: why two individuals who both carry the same virus can have wildly different experiences with it.
Treatment and Prevention
There is no way to eliminate HSV-1 from the body once it has established latency. Current treatments focus on reducing the severity of outbreaks and lowering the frequency of recurrences.
Antiviral medications like acyclovir, valacyclovir, and famciclovir work by blocking viral replication. When started early in an outbreak, they shorten the duration of symptoms and help lesions heal faster.15PubMed Central. Pharmacy-Based Assessment and Management of Herpes Labialis (Cold Sores) with Antiviral Therapy Topical formulations applied directly to the sore are available over the counter in many countries, though oral antivirals tend to be more effective, especially for severe or frequent outbreaks.
For people with frequent recurrences, daily suppressive therapy with oral antivirals can reduce the number of outbreaks over time. The Cochrane review on prevention found that long-term use of oral acyclovir roughly halved the number of clinical recurrences over a four-month period and had an even greater effect on virological recurrence (detectable viral episodes). Valacyclovir showed a similar preventive benefit.8PubMed Central. Interventions for prevention of herpes simplex labialis Antiviral treatment can also reduce viral shedding in saliva. In a study of Antarctic expeditioners, those given valacyclovir had substantially less HSV-1 DNA in their saliva than those given a placebo, though the reduction did not quite reach statistical significance due to small sample size.16Frontiers in Virology. Antiviral treatment with valacyclovir reduces virus shedding in saliva of Antarctic expeditioners
Beyond antivirals, practical trigger avoidance matters. Using lip balm with SPF when you know you will be in the sun, managing stress with sleep and basic self-care, and giving your dentist a heads-up about your cold sore history before invasive procedures are all reasonable steps. None of these guarantee prevention, but each removes one of the known nudges that tip the virus toward reactivation.
A Virus That Has Been With Us for Millions of Years
HSV-1 is not a recent acquisition for our species. Phylogenetic analysis suggests that it has been co-evolving with human ancestors for millions of years, tracing back at least to the common ancestor of Old World monkeys and apes. HSV-2, by contrast, appears to have jumped to an extinct human ancestor from the lineage that became modern chimpanzees roughly 1.6 million years ago.17Oxford Academic (Molecular Biology and Evolution). Evolutionary origins of human herpes simplex viruses 1 and 2
This deep evolutionary history explains some of the virus’s behavior. HSV-1 has had an extraordinarily long time to fine-tune its ability to hide from the immune system, establish stable latency without killing the host, and reactivate just often enough to find new hosts. The result is a pathogen that is genuinely difficult to eliminate and that infects the majority of the global adult population. Vaccine development has been underway for decades with limited success so far, in part because the virus’s latency program makes it invisible to vaccine-primed immune responses once it reaches the ganglion. Research continues on several fronts, including gene-editing approaches that aim to disable the latent viral genome directly within nerve cells, but a broadly effective vaccine or cure remains elusive.
Cold Sores Versus Canker Sores
A persistent source of confusion is the difference between cold sores and canker sores. Cold sores are caused by HSV-1, appear on or around the lips, start as fluid-filled blisters that eventually crust over, and are contagious. Canker sores (aphthous ulcers) are shallow, painful ulcers that form inside the mouth, on the gums, inner cheeks, or tongue. They are not caused by a virus, are not contagious, and their exact cause is unclear, though stress, minor mouth injuries, acidic foods, and immune dysfunction are thought to play a role.
The two conditions look and feel different once you know what to look for. Cold sores typically start with a tingling or burning sensation on the lip, followed by a cluster of small blisters that merge and then crust. Canker sores are flat, round or oval ulcers with a white or yellow center and a red border, found exclusively inside the mouth. If a sore is on the outside of your lip and started as a blister, it is almost certainly a cold sore. If it is inside your mouth and never blistered, it is more likely a canker sore. The distinction matters practically because antivirals work on cold sores but have no effect on canker sores, and misidentifying one for the other wastes time and money on the wrong treatment.