Cold sores are caused by herpes simplex virus type 1 (HSV-1), and you get them through direct contact with the virus, most commonly by kissing or touching the skin around someone’s mouth during an active outbreak. The virus can also spread through saliva and shared objects, and it often passes between people when no visible sore is present. Most people pick up HSV-1 in childhood or early adolescence from a family member, and once infected, you carry the virus for life.
How HSV-1 Spreads From Person to Person
HSV-1 travels through direct contact with infected skin, sores, or saliva. Kissing is the most common route, but the virus can also spread by sharing lip balm, utensils, razors, or towels. The risk is highest when someone has an active blister, but both HSV-1 and HSV-2 can be transmitted when the skin looks completely normal. This is because the virus periodically reaches the skin surface and sheds without producing any visible symptoms.
Research from the University of Washington tracked how often people shed the virus without knowing it. At two months after infection, participants shed HSV-1 on about 12% of days. By 11 months, that dropped to 7% of days. In most of those instances, the person had no symptoms at all. This silent shedding is a major reason the virus is so widespread: people pass it along without realizing they’re contagious.
HSV-1 can also survive on dry surfaces for anywhere from a few hours to several weeks, with longer survival at lower humidity. That said, transmission from objects is far less common than skin-to-skin contact. The virus needs to reach a mucous membrane or a break in the skin to establish infection.
What Happens Inside Your Body After Exposure
When HSV-1 lands on the thin, moist skin of your lips or the lining of your mouth, it latches onto the surface of epithelial cells using proteins on its outer shell. The virus then fuses with the cell membrane and injects its genetic material inside. Once in, it hijacks the cell’s machinery to make copies of itself, which quickly spread to neighboring cells.
The incubation period, the gap between exposure and your first symptoms, ranges from 1 to 26 days but is typically 6 to 8 days. A first outbreak is often the most severe. Some people develop painful blisters, swollen gums, fever, and sore throat. Others have such mild symptoms they never realize they were infected.
After the initial infection clears, the virus doesn’t leave. It travels along nerve fibers to a cluster of nerve cells near the base of the skull called the trigeminal ganglion. There it enters a dormant state called latency. During latency, the virus is essentially silent. It produces almost no proteins, making it invisible to your immune system and untouchable by antiviral medications. Your immune system stations specialized T cells at the ganglion to keep the virus in check, but it cannot eliminate it.
Why Cold Sores Keep Coming Back
Reactivation happens when something disrupts the balance between the dormant virus and the immune cells guarding it. Researchers at the University of Virginia found that the virus reactivates when the neurons housing it become overstimulated, a state called neuronal hyperexcitation. The virus detects this change and uses the opportunity to begin replicating again. It then travels back down the nerve fibers to the skin surface, where it produces a new sore.
Common triggers that set off this process include:
- Sunlight and UV exposure: sunburn on or around the lips is one of the most reliable triggers
- Physical or emotional stress: anything that taxes the immune system or overstimulates the nervous system
- Illness or fever: colds, flu, and other infections (which is why they’re called “fever blisters”)
- Hormonal changes: menstruation is a well-documented trigger for some women
- Fatigue and sleep deprivation
- Skin trauma: dental procedures, facial surgery, or windburn around the lips
Not everyone who carries HSV-1 gets frequent outbreaks. Some people have one or two episodes and never see another sore. Others deal with several flare-ups a year, especially in the first year or two after infection. Outbreak frequency generally decreases over time as the immune system builds a stronger response at the nerve site.
Cold Sores vs. Canker Sores
Many people confuse cold sores with canker sores, but they are entirely different conditions. The easiest 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 form inside the mouth, on the inner cheeks, tongue, or soft palate.
They also look different. Cold sores start as a cluster of small, fluid-filled blisters that eventually break open, weep, and crust over. Canker sores are usually a single round sore with a white or yellow center and a red border. Canker sores are not caused by a virus and are not contagious.
HSV-1 vs. HSV-2
Cold sores are overwhelmingly caused by HSV-1, but HSV-2, the strain more commonly associated with genital herpes, can occasionally cause oral sores too. This typically happens through oral sex with someone who has a genital HSV-2 infection. HSV-2 oral infections tend to recur less frequently than HSV-1 oral infections because HSV-2 “prefers” the genital region, just as HSV-1 preferentially establishes itself around the mouth.
The reverse is also true: HSV-1 can cause genital herpes if transmitted through oral sex. Both strains can infect either location, but each tends to be more active and recur more often in its preferred site.
Reducing the Risk of Transmission
Because the virus spreads through direct contact and saliva, avoiding kissing or sharing personal items with someone who has an active sore is the most effective precaution. If you carry the virus, keeping your hands away from an active blister and washing your hands if you do touch one helps prevent spreading it to your eyes or other parts of your body.
Since asymptomatic shedding accounts for a significant share of transmission, complete prevention is difficult. Many people contract HSV-1 during childhood from a parent’s kiss, long before anyone thinks to take precautions. Using lip balm with SPF can help prevent sun-triggered outbreaks, and managing stress and sleep may reduce how often the virus reactivates. Antiviral medications can shorten outbreaks and reduce shedding, but they do not eliminate the virus from the body.