Fever blisters are caused by herpes simplex virus type 1 (HSV-1), a highly common infection carried by an estimated 3.8 billion people under age 50, or about 64% of the global population. The virus spreads through direct contact with sores, saliva, or skin around the mouth, and once you’re infected, it stays in your body permanently. What most people experience as a fever blister is actually a reactivation of a virus that’s been quietly living in their nerve cells, sometimes for years.
How the Virus Gets In and Stays
HSV-1 spreads through kissing, sharing utensils or lip products, or any skin-to-skin contact with an infected area. Many people contract it during childhood from a parent or caregiver. The first infection may cause no noticeable symptoms at all, or it can trigger a more severe initial outbreak with sores inside the mouth, sore throat, and swollen glands.
After that first infection, the virus travels along nerve fibers and settles into a cluster of nerve cells near the base of the skull called the trigeminal ganglion. There, it essentially goes to sleep. The virus shuts down its active replication machinery and hides inside neurons, converting them into a lifelong reservoir. Your immune system can’t clear it from these nerve cells, which is why the infection is permanent.
When something disrupts the balance, the virus reactivates. It travels back down the nerve fibers to the skin surface around the lips, where it begins replicating again and produces the familiar blister.
What Triggers a Flare-Up
Not everyone who carries HSV-1 gets frequent fever blisters. Some people have one outbreak and never another, while others deal with several per year. The difference comes down to individual immune function and exposure to triggers that wake the dormant virus. Common triggers include:
- Stress: Prolonged psychological stress prompts your immune system to release an inflammatory signal called interleukin-1 beta. Researchers at UVA Health found that this molecule increases the excitability of the neurons harboring the virus, essentially giving HSV-1 the green light to reactivate. The virus has evolved to hijack this immune response, using what should be infection-fighting machinery to its own advantage.
- Sunlight: Ultraviolet light damages skin cells on and around the lips, which also triggers the release of that same inflammatory signal. This is why many people notice fever blisters after a day at the beach or on a ski slope.
- Illness or fatigue: A cold, the flu, or anything that taxes your immune system can loosen the controls keeping the virus dormant.
- Hormonal changes: Menstruation is a well-recognized trigger for some women.
- Physical trauma to the lips: Dental work, cosmetic procedures, or even chapped, windburned lips can set off an outbreak.
The underlying mechanism is the same across these triggers: stress on the neurons or suppression of the local immune response allows the virus to switch from its dormant state back into active replication.
What a Fever Blister Looks and Feels Like
Each outbreak follows a predictable pattern over about 7 to 10 days. It starts with a tingling, itching, or burning sensation on or around the lip, often called the prodromal stage. This is the earliest warning sign, and it’s the window when treatment is most effective. Within a day or two, one blister or a tight cluster of small blisters forms, usually at the border of the lip.
A few days later, the blisters burst and drain fluid. This is the most contagious phase. The open sore then crusts over into a scab, which gradually heals and falls off, leaving clear skin behind. Scarring is rare unless the area gets infected with bacteria during the healing process.
How Fever Blisters Spread to Others
The virus is most contagious when blisters are open and weeping, but it can also spread when no sores are visible. This is called asymptomatic shedding, meaning the virus is present on the skin surface without causing symptoms. Research from the University of Washington found that people with HSV-1 shed the virus on about 7 to 12% of days in the months following infection, with rates declining over time. Some people shed virus on as few as 1% of days after a couple of years.
This means you can pass the virus to someone through a kiss even when your skin looks completely normal. It also means avoiding contact only during visible outbreaks reduces transmission risk but doesn’t eliminate it entirely.
When the Virus Spreads to Other Body Parts
HSV-1 occasionally travels beyond the lips. If you touch an active blister and then rub your eye, the virus can infect the cornea, a condition called herpes keratitis. The CDC notes that most cases of herpes eye infections happen after another part of the body, usually the mouth, has already been infected. Thorough handwashing before touching your eyes during an outbreak is the simplest way to prevent this.
The virus can also infect the fingers if you pick at a blister, causing painful sores on the fingertips. And while HSV-1 primarily causes oral infections, it can be transmitted to the genital area through oral-genital contact.
Treatment and Prevention
Antiviral medications work best when taken at the very first sign of tingling or burning, before a blister forms. Prescription antivirals can shorten an outbreak to roughly one day of treatment if started early enough. For people who get frequent outbreaks, daily suppressive therapy can reduce the number of flare-ups per year.
Over-the-counter options like topical creams containing docosanol can modestly reduce healing time but are less effective than prescription antivirals. Pain relief during an outbreak is straightforward: over-the-counter pain relievers, ice applied to the area, and avoiding acidic or salty foods that irritate the sore.
On the prevention side, wearing lip balm with SPF protection helps block UV-triggered outbreaks. Managing stress through sleep, exercise, and regular downtime addresses one of the most common reactivation triggers. Some people find that the amino acid L-lysine, taken as a supplement, reduces outbreak frequency. A six-month clinical trial found that participants taking L-lysine had 2.4 times fewer outbreaks than those on a placebo, with shorter healing times and less severe symptoms. However, doses below 1 gram per day appear to be ineffective. Studies suggesting benefit used doses of 3 grams or more daily.
Arginine, an amino acid found in chocolate, nuts, and seeds, appears to support viral replication. Some people find that reducing high-arginine foods during prodromal symptoms helps, though the evidence for dietary management is less robust than for antiviral medication.