Cold sores are caused by herpes simplex virus type 1 (HSV-1), and you get it through direct contact with an infected person’s skin, saliva, or sores. Most people pick up the virus during childhood or young adulthood through nonsexual contact, often from a kiss by a parent or relative. Once you’re infected, the virus stays in your body permanently and can reactivate to produce new sores throughout your life.
How HSV-1 Spreads
The virus primarily spreads through skin-to-skin contact. Kissing someone who carries HSV-1 is the most common route, but any contact with an infected person’s saliva or the skin around their mouth can transmit it. The greatest risk comes when someone has an active sore, but transmission can also happen when the skin looks completely normal. This is because the virus can be present on the surface of the skin or in saliva even between outbreaks, a process called asymptomatic shedding.
You can also get genital herpes from HSV-1 if you receive oral sex from someone with an oral infection. This has become an increasingly recognized route of transmission, particularly among young adults.
One thing you don’t need to worry about: picking up the virus from objects. The CDC is clear that you will not get herpes from toilet seats, bedding, swimming pools, silverware, soap, or towels. The virus doesn’t survive well on surfaces.
What Happens After Exposure
After HSV-1 enters through your skin or mucous membranes, it typically takes six to eight days for symptoms to appear, though the incubation period can range anywhere from one to 26 days. Many people never develop noticeable symptoms at all, which is why the virus spreads so easily. Carriers who don’t know they’re infected account for a large share of transmission.
Your first infection, if it does cause symptoms, tends to be the worst one. A primary outbreak can include fever, headache, muscle aches, and sore, swollen gums (a condition called gingivostomatitis that’s common in children). The acute illness lasts about five to seven days, with full healing taking up to two weeks. Recurrent cold sores, by contrast, are typically milder and shorter.
Why the Virus Never Leaves
After your first infection, HSV-1 travels along nerve fibers to a cluster of nerve cells near your temple called the trigeminal ganglion. There, the viral DNA tucks itself into the nucleus of neurons and goes quiet. It doesn’t actively replicate. It doesn’t produce symptoms. It just sits there, essentially invisible to your immune system, for the rest of your life.
This is why there’s no cure for cold sores. The dormant virus hides in a place your body can’t clear it from. It can reactivate at any time, traveling back down the nerve fibers to the skin surface to produce a new sore.
What Triggers a Recurrence
Researchers at the University of Virginia identified the biological mechanism behind cold sore flare-ups: when neurons harboring the dormant virus become overstimulated, the virus senses the change and reactivates. Stress, illness, and sunburn all trigger this chain of events through a common pathway. The immune system releases an inflammatory signal molecule during prolonged stress or illness, and that same molecule is released by skin cells damaged by ultraviolet light. It increases excitability in the affected neurons, giving the virus its window to wake up.
The practical triggers people notice most often include:
- Physical stress: fever, illness, surgery, or dental work
- Emotional stress: anxiety, sleep deprivation, or major life changes
- Sun exposure: UV light on the lips, especially without sunscreen
- Hormonal changes: menstruation is a commonly reported trigger
- A weakened immune system: from medication, illness, or fatigue
Not everyone who carries HSV-1 gets frequent cold sores. Some people have one outbreak and never another. Others deal with several per year. The pattern varies widely from person to person and often becomes less frequent over time.
Spreading the Virus to Yourself
If you have an active cold sore, you can transfer the virus to other parts of your own body. The most concerning example is herpes keratitis, an eye infection. The CDC notes that most cases of herpes-related eye infection happen after another part of the body, most commonly the mouth, has already been infected. Touching a cold sore and then rubbing your eye is the typical route. Herpetic whitlow, a painful infection of the fingers, can happen the same way.
Washing your hands thoroughly before touching your eyes is the single most important precaution during an active outbreak. Avoid picking at or touching the sore, and if you do, wash your hands immediately.
How to Reduce Your Risk
Because HSV-1 can spread even when no sore is visible, completely preventing transmission is difficult. But you can significantly lower the odds. Avoid kissing or sharing close facial contact with someone who has an active cold sore. The first 24 hours of a visible outbreak carry the highest concentration of virus, though shedding can continue for about five days.
If you already carry the virus and want to protect others, avoid kissing or oral contact during outbreaks. Wearing lip balm with SPF can help prevent sun-triggered recurrences. Managing stress through sleep, exercise, or other strategies may reduce flare-up frequency, since stress is one of the most reliable reactivation triggers.
For people who get frequent outbreaks, antiviral medications can shorten the duration of a sore and reduce how often they occur. Starting treatment at the first sign of tingling or burning, before the blister fully forms, makes these medications most effective.