Fever blisters are caused by herpes simplex virus type 1 (HSV-1), a virus carried by roughly 3.8 billion people worldwide, or about 64% of the global population under age 50. The virus never leaves your body once you’re infected. It hides in nerve cells near the base of your skull and periodically reactivates, producing the painful blisters that appear on or around the lips.
How the Virus Gets Into Your Body
HSV-1 spreads through skin-to-skin contact, most commonly through kissing or sharing items that touch the mouth. Most people pick up the virus during childhood through nonsexual contact with a family member or caregiver. The virus can also spread through oral-genital contact, which is why HSV-1 is now a growing cause of genital herpes as well.
What makes HSV-1 especially easy to transmit is that it doesn’t require a visible sore. In one study, 70% of new infections were traced to contact during periods when the carrier had no symptoms at all. The virus can shed from the skin surface without producing any blisters, meaning many people pass it on without ever knowing they’re contagious. Shedding without symptoms is most frequent soon after someone first catches the virus.
Where the Virus Hides Between Outbreaks
After the initial infection, HSV-1 travels along nerve fibers and settles into a cluster of nerve cells called the trigeminal ganglia, located near the jawline. There, the viral DNA enters the nucleus of neurons but doesn’t merge with your own DNA. Instead, it forms a small, separate loop of genetic material that your immune system can’t easily detect or destroy.
In this dormant state, the virus goes nearly silent. Of its roughly 80 genes, almost all shut down. Only a small family of non-coding RNA molecules continues to be produced. These transcripts help maintain the virus in its quiet, hibernation-like state, keeping it alive inside the nerve cell without triggering an immune response. This is why the infection is lifelong: the virus isn’t replicating, so antiviral drugs can’t reach it, and the immune system can’t see it clearly enough to eliminate it.
What Triggers an Outbreak
A fever blister appears when something disrupts that dormant state and the virus begins replicating again. It travels back down the nerve fibers to the skin surface, where it causes the characteristic blister. Several well-documented triggers can set off this process.
Fever and Illness
Fever is one of the oldest and most strongly associated triggers, which is exactly how fever blisters got their name. When your body temperature rises, the heat itself can stress the neurons harboring the virus. On top of that, the inflammatory molecules your body releases during a fever, including certain cytokines and prostaglandins, can act directly on nerve cells and push the virus out of dormancy.
Sunlight and UV Exposure
Ultraviolet radiation is a potent reactivation trigger. In a controlled trial, 71% of participants with a history of fever blisters developed a new outbreak after UV exposure to the lip area. UV light damages cells at the skin surface, which sends distress signals back along the nerve. These signals alter levels of neuropeptides, neurotransmitters, and other molecules in ways that can wake the dormant virus.
Psychological Stress
Emotional stress activates your body’s fight-or-flight system, flooding the bloodstream with stress hormones like adrenaline and cortisol. Both of these chemicals have been shown to trigger HSV-1 reactivation in laboratory and animal studies. Chronic stress may also reduce the supply of nerve growth factors that help keep the virus suppressed, giving it an opening to reactivate.
Hormonal Changes
Menstruation is a recognized trigger for many women, likely because shifts in hormone levels affect the same nerve growth factor pathways that help keep the virus dormant. Other hormonal fluctuations, including those during pregnancy or around menopause, may have a similar effect.
Physical Trauma to the Area
Dental procedures, facial surgery, or any physical trauma near the mouth or along the trigeminal nerve can trigger an outbreak. Surgical treatments for trigeminal neuralgia, for example, are well documented to cause reactivation. The mechanism is straightforward: damaging the nerve fibers where the virus lives activates cellular stress responses that disrupt the virus’s dormant state.
HSV-2 as a Less Common Cause
While HSV-1 is responsible for the vast majority of fever blisters, HSV-2 (typically associated with genital herpes) can occasionally cause oral lesions. In a study of nearly 1,400 people who carried both virus types, oral HSV-2 shedding was detected far less frequently than oral HSV-1 shedding (0.06% versus 1% of days sampled). When HSV-2 does appear in the mouth, it usually happens during a first episode of genital HSV-2 infection or during an active genital recurrence, not as an independent oral outbreak.
What a Fever Blister Outbreak Looks Like
Outbreaks follow a predictable pattern. The first sign is a tingling, itching, or burning sensation at the spot where the blister will form. This prodrome stage lasts several hours to about a day. A fluid-filled blister then develops, typically on the lip border or just outside the mouth. Within about 48 hours, the blister ruptures, oozes clear fluid (which is highly contagious), and begins to crust over into a scab. From start to finish, most fever blisters heal within 5 to 15 days.
The first outbreak a person ever experiences tends to be the most severe, sometimes accompanied by fever, swollen lymph nodes, and sore throat. Subsequent outbreaks are usually milder and shorter. Some people have multiple recurrences per year, while others go years or even decades between episodes.
Antiviral Treatment
Prescription antiviral medications can shorten an outbreak, especially when taken at the first sign of tingling. Valacyclovir, one of the most commonly prescribed options, is taken as a one-day course for fever blisters. These medications work by blocking the virus from replicating once it reactivates, so they’re most effective the earlier you start them. They don’t eliminate the dormant virus from your nerve cells, which is why outbreaks can still return.
Over-the-counter creams containing docosanol can also reduce healing time by a day or so when applied early, though they’re generally less effective than prescription antivirals.
Preventing Outbreaks
Since you can’t remove the virus once it’s established, prevention focuses on avoiding known triggers. Sunscreen on the lips is one of the most evidence-backed strategies. In the same UV exposure trial mentioned earlier, not a single participant who applied sunscreen before UV exposure developed a fever blister, compared to 71% who used a placebo. A lip balm with SPF 30 or higher, reapplied regularly during sun exposure, is a simple and effective measure.
Managing stress through sleep, exercise, and other lifestyle habits may reduce the frequency of outbreaks, though the effect is harder to quantify than UV protection. Keeping your immune system in good shape generally helps: people who are immunocompromised tend to experience more frequent and more severe recurrences.
Complications Worth Knowing About
For most people, fever blisters are a painful nuisance and nothing more. But the virus can occasionally spread to other parts of the body, and two locations deserve particular attention.
Touching an active blister and then rubbing your eye can transfer the virus to the cornea, causing ocular herpes. Symptoms include eye redness, pain, light sensitivity, watery eyes, and a feeling like something is stuck in your eye. Ocular herpes is serious and can cause vision loss if not treated promptly. Touching an active sore and then a cut on your finger can also cause herpetic whitlow, a painful infection of the fingertip.
The simplest way to avoid spreading the virus to other body parts, or to other people, is to avoid touching active blisters and to wash your hands thoroughly if you do.