Fever blisters, also called cold sores, come from herpes simplex virus type 1 (HSV-1), a virus carried by roughly 64% of the global population under age 50. Most people pick up the virus during childhood through ordinary close contact, and it stays in the body permanently. Once you have it, certain triggers can wake the virus up and cause a blister to form on or around your lips.
How the Virus Gets Into Your Body
HSV-1 spreads through direct skin-to-skin contact or contact with saliva. A parent kissing a child, sharing a drink, or using the same lip balm can all pass the virus along. The infection is most contagious when someone has a visible sore, but the virus can also be present on the skin with no symptoms at all. Studies tracking viral shedding found that people with HSV-1 shed the virus on about 7 to 12% of days in the months after infection, even without any sores present. Over time, shedding drops significantly.
Because transmission is so easy and so common, most people contract HSV-1 before adulthood without ever realizing it. Many never develop a visible fever blister. Others get occasional outbreaks triggered by specific conditions.
What the Virus Does After Infection
After the initial infection, HSV-1 travels along nerve fibers and settles into a cluster of nerve cells near the base of the skull called the trigeminal ganglion. There, it enters a dormant state. During this dormancy, the virus produces almost no detectable proteins and essentially hides from your immune system. It can stay silent for months, years, or an entire lifetime.
When something disrupts your body’s equilibrium, the virus can reactivate. It travels back down the nerve fibers to the skin surface, where it begins replicating and produces the characteristic blister.
Common Triggers That Cause an Outbreak
HSV-1 is opportunistic. It reactivates when your immune system is weakened or your body is under stress. The triggers fall into several categories.
Illness and fever. This is where the name “fever blister” comes from. A common cold, the flu, or any infection that taxes your immune system can open the door for an outbreak. Fevers themselves are a frequent trigger.
Sun exposure. Ultraviolet light is one of the most well-documented reactivation triggers. UV radiation activates a stress receptor found in about half of the nerve cells where the virus hides. This receptor can switch on key viral genes, pulling the virus out of dormancy even without direct damage to the lips. Sunburns on or around the mouth are especially risky.
Physical and emotional stress. Chronic stress, sleep deprivation, and intense physical exertion all suppress immune function. Interestingly, the same cellular stress receptor activated by UV light also responds to stress hormones, which helps explain why emotional stress and sunlight both lead to the same result.
Hormonal shifts. Menstruation, pregnancy, puberty, and menopause can all trigger outbreaks. Hormonal fluctuations seem to create a window of vulnerability the virus exploits.
Lip injury or trauma. Any damage to the lips, including bruises, cuts, cracked skin from cold weather, or even cosmetic procedures like filler injections, can provoke an outbreak. The inflammatory response from the injury appears to be the mechanism.
Extreme temperatures. Both intense heat and bitter cold stress the body and dry out the skin on the lips, creating favorable conditions for reactivation.
Weakened immunity. Autoimmune conditions, cancer treatments, and immunosuppressive medications all raise the risk of more frequent and more severe outbreaks.
What a Fever Blister Looks and Feels Like
A cold sore progresses through distinct stages over the course of 5 to 15 days.
The first sign is a tingling, itching, or burning sensation on the lip, usually several hours to a full day before anything is visible. This is called the prodromal stage, and it’s the most important window for treatment. Next, the skin in that area becomes discolored and swollen, forming a small raised bump. Within a day or so, fluid-filled blisters appear, typically clustered on one side of the lip.
About 48 hours after the blisters form, they break open and ooze clear fluid. This is the most contagious and often the most painful stage. A yellowish crust then forms over the raw area. The scab may crack and bleed as you talk or eat, but it gradually tightens as the skin underneath heals. Eventually the scab falls off, leaving skin that may be slightly pink for a few more days before returning to normal.
Reducing the Frequency and Severity
Since you can’t eliminate the virus from your body, management focuses on avoiding triggers and treating outbreaks early. Wearing lip balm with SPF protection year-round is one of the simplest preventive steps, especially if sun exposure is a known trigger for you. Managing stress, getting consistent sleep, and keeping your lips moisturized in cold weather all help reduce the odds of reactivation.
Antiviral medications work best when taken at the very first sign of tingling, before a blister forms. Starting treatment during the prodromal stage can shorten the outbreak by several days or, in some cases, prevent the blister from fully developing. Your doctor can prescribe antiviral pills for episodic use, meaning you keep them on hand and take them only when you feel an outbreak starting. For people who get frequent outbreaks, daily suppressive therapy is sometimes an option, though this is more commonly used for genital herpes than for oral cold sores.
Over-the-counter creams containing the antiviral docosanol can also help if applied early, though they tend to be less effective than prescription options.
Preventing Spread to Other Body Parts
The same virus that causes a fever blister on your lip can infect other areas if you transfer it with your hands. Touching an active sore and then rubbing your eye can lead to a serious eye infection called herpes keratitis, which causes pain, redness, blurred vision, light sensitivity, and watery discharge. Left untreated, it can scar the cornea and impair vision permanently.
The simplest protection is thorough handwashing, especially after touching your face during an active outbreak. Avoid rubbing your eyes, and don’t share towels, razors, or lip products while you have a visible sore. Contact lens wearers should be particularly careful, since the virus can transfer to the lens and then to the eye.
Why Some People Get Outbreaks and Others Don’t
Most people carrying HSV-1 never get a single fever blister. The reasons aren’t fully understood, but genetics play a role in how effectively the immune system keeps the virus suppressed. Some people’s immune cells are better at patrolling the nerve clusters where the virus hides. Research has found that specialized immune cells remain stationed in the trigeminal ganglion specifically to monitor for viral reactivation, and the strength of this surveillance varies from person to person.
If you get frequent outbreaks, it doesn’t mean your immune system is failing. It likely reflects a combination of genetic susceptibility, exposure to triggers, and the particular strain of virus you carry. Tracking your personal triggers, whether that’s stress, sun, hormonal cycles, or illness, gives you the best chance of reducing how often they occur.