Cold sores are caused by herpes simplex virus type 1 (HSV-1), a virus carried by roughly 48% of Americans between ages 14 and 49. Once you’re infected, the virus never leaves your body. It hides in nerve cells near your jaw and reactivates periodically, producing the painful blisters that show up on or around your lips. Understanding both the initial infection and the triggers that wake the virus up is key to making sense of why cold sores keep coming back.
How HSV-1 Infects and Hides
HSV-1 typically enters through the moist tissue inside your mouth, most often the inner lip or cheek lining. Once inside, it hijacks your skin cells, forces them to produce copies of itself, and destroys them in the process. That burst of viral replication is what produces the first cold sore outbreak.
But the virus doesn’t stop there. It slips into the nerve endings in your skin and travels backward along the nerve fibers to a cluster of nerve cells called the trigeminal ganglion, located near the base of your skull. Inside those neurons, the viral DNA settles into the cell’s nucleus without merging into your own genetic code. It essentially goes quiet, producing almost no viral proteins. Your immune system can’t detect it in this dormant state, which is why the infection lasts a lifetime.
When something disrupts the balance (stress, illness, sun exposure), the virus wakes up, makes new copies of itself, and travels back down the same nerve fibers to the skin around your lips. That return trip is what produces a recurrent cold sore, sometimes in the exact same spot as previous outbreaks.
How the Virus Spreads
You catch HSV-1 through direct contact with an infected person’s skin or saliva. Kissing is the most common route, but sharing utensils, lip balm, or razors can also transmit it. The virus spreads most easily when an active sore is present, but it can also be transmitted when the skin looks completely normal. This “silent shedding” is one reason HSV-1 is so widespread: people pass it on without knowing they’re contagious.
Most people contract HSV-1 during childhood, often from a parent or close relative. Many never develop noticeable symptoms from the initial infection, which means they may carry the virus for years before their first visible cold sore, if they ever get one at all.
Triggers That Reactivate the Virus
A cold sore outbreak isn’t a new infection. It’s the dormant virus waking up. Several well-known triggers can set off that reactivation, and most of them share a common thread: they either weaken your immune system or damage the skin around your lips.
Stress
Both emotional and physical stress are among the most reliable triggers. In the short term, stress floods your body with cortisol and adrenaline, which suppress parts of the immune response. Chronic stress is even more problematic because it creates persistent low-grade inflammation. When your immune system is busy managing that background inflammation, it’s less effective at keeping HSV-1 in check.
Sun Exposure and Extreme Temperatures
Ultraviolet light is a well-documented trigger. Sunburn on or around the lips damages skin cells and sparks an inflammatory response that can open the door for reactivation. Extreme cold is similarly problematic. It dries and cracks the skin on your lips, creating physical damage that the virus can exploit. Both very hot and very cold weather put stress on your body that tips the balance in the virus’s favor.
Illness and Fatigue
Cold sores earned the nickname “fever blisters” for a reason. When your immune system is already occupied fighting the flu, a cold, or another infection, it has fewer resources to keep HSV-1 suppressed. The same logic applies to exhaustion. Running on low sleep and depleted energy taxes your immune defenses, making outbreaks more likely.
Hormonal Changes
Hormonal fluctuations during the menstrual cycle are a common trigger for women. Many people notice cold sores appearing just before or during their period. The shifting hormone levels appear to temporarily alter immune function enough to let the virus reactivate.
Skin Damage
Sunburns aren’t the only type of skin injury that matters. Rashes, severe acne, cuts, scrapes, or even dental procedures that irritate the area around your mouth can trigger an inflammatory response that gives HSV-1 an opening.
What a Cold Sore Outbreak Looks Like
A typical cold sore moves through five stages over the course of about 7 to 10 days. It starts with a tingling, burning, or itching sensation on or near your lip, usually a day or two before anything is visible. This prodromal stage is actually the best window for treatment, since antiviral medications work most effectively when started early.
Next, one or more small, fluid-filled blisters appear on red, inflamed skin. Within a few days, the blisters rupture and become open, shallow sores. This “weeping” stage is the most contagious and often the most painful. The open sore then dries out and forms a yellowish or brown crust. Finally, the scab flakes away as new skin forms underneath. Some people get outbreaks several times a year, while others may have one episode and never experience another.
Who Gets More Frequent or Severe Outbreaks
Anyone carrying HSV-1 can get cold sores, but certain people face a higher burden. Those with weakened immune systems, whether from a medical condition or medications that suppress immunity, tend to have more frequent and more severe outbreaks. People with moderate to severe eczema are at particular risk. In them, the virus can spread beyond the usual lip area and cause a serious, widespread skin infection that requires urgent medical treatment. Infants and young children with eczema are especially vulnerable.
Frequency also varies for reasons that aren’t fully understood. Some people’s immune systems simply keep the virus in check more effectively than others, which is why two people with the same triggers can have very different outbreak patterns.
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 outbreaks too. CDC data from 2015 to 2016 found HSV-1 prevalence at about 48% in Americans aged 14 to 49, compared to about 12% for HSV-2. Both strains are lifelong infections, and both can be transmitted with or without visible symptoms. Oral HSV-2 outbreaks tend to recur less frequently than oral HSV-1 outbreaks, but the sores themselves look identical.
How Cold Sores Are Managed
There’s no cure for HSV-1, but antiviral medications can shorten outbreaks and reduce how often they happen. Oral antivirals work best when taken at the first sign of tingling, before blisters form. For people who get frequent outbreaks (six or more per year, for example), daily suppressive therapy can significantly reduce recurrence. Topical antiviral creams are also available over the counter, though they’re generally less effective than oral medications.
Beyond medication, managing your known triggers makes a real difference. Wearing lip balm with SPF, getting adequate sleep, and finding ways to manage chronic stress can all help keep the virus dormant for longer stretches. Keeping your lips moisturized in cold, dry weather reduces the skin damage that invites reactivation. None of these strategies eliminate the virus, but they can meaningfully reduce how often it disrupts your life.