Cold sores are caused by herpes simplex virus type 1 (HSV-1), a highly common infection carried by roughly 3.8 billion people under age 50, or about 64% of the global population. The virus enters skin cells around the mouth, establishes a permanent presence in your body, and periodically reactivates to produce the blisters most people recognize as cold sores.
How the Virus Gets In
HSV-1 spreads through direct skin-to-skin contact or saliva. The most common routes are kissing someone who carries the virus, sharing utensils or lip balm, or receiving oral sex from someone with genital herpes. You don’t need to see a visible sore for transmission to happen. In fact, research on herpes transmission found that 70% of new infections were traced to contact during periods when the carrier had no symptoms at all. This “asymptomatic shedding,” where the virus is active on the skin surface without producing blisters, is the main way HSV-1 quietly passes between people.
Once the virus reaches the skin, it targets keratinocytes, the primary cells in your outer skin layer. Four viral proteins latch onto receptors on those cells and allow the virus inside. From there, HSV-1 can spread directly from cell to cell without needing to re-enter from outside, which helps it establish a foothold before your immune system fully responds.
Why the Virus Never Leaves
After the initial infection, HSV-1 travels along nerve fibers and settles into a cluster of nerve cells called the trigeminal ganglion, located near your jaw. There it enters a dormant state called latency. The virus can remain silent in these neurons for months, years, or even a lifetime without causing symptoms.
Your immune system plays an active role in keeping it quiet. Specialized immune cells (CD8+ T cells) camp out in the ganglion and monitor for signs of viral activity. When they detect neurons starting to produce small amounts of viral protein, they release molecules that shut the process down before it can develop into a full outbreak. This constant surveillance is why many carriers go long stretches without cold sores, and why some never develop visible symptoms at all.
What Triggers a Flare-Up
When that immune surveillance weakens or the nervous system is stressed, the virus can reactivate. Researchers at the University of Virginia found that the key mechanism involves “neuronal hyperexcitation,” a state where the nerve cells harboring the virus become overstimulated. The virus senses this change and uses the opportunity to wake up, travel back down the nerve fibers to the skin surface, and begin replicating.
Several well-documented triggers push the body into that vulnerable state:
- UV exposure and sunburn: Direct sunlight on the lips is one of the most reliable triggers, which is why cold sores often appear after a day at the beach or on a ski trip.
- Physical illness or fever: Any infection that taxes the immune system can open the door. Cold sores earned their name because they frequently accompany common colds.
- Psychological stress: Chronic or acute stress raises levels of hormones that suppress immune function, weakening the T cells that keep the virus in check.
- Fatigue and sleep deprivation: Both reduce immune surveillance in the nerve ganglion.
- Hormonal shifts: Menstruation is a commonly reported trigger, likely due to fluctuations in immune activity across the cycle.
- Skin trauma: Dental procedures, cosmetic treatments around the lips, or any physical injury to the area can provoke reactivation.
HSV-2 Can Also Cause Cold Sores
While HSV-1 is responsible for the vast majority of oral herpes cases, HSV-2 (the type more commonly associated with genital herpes) can also infect the mouth. This typically happens through oral sex with a partner who has a genital HSV-2 infection. The resulting cold sores look and feel identical, though oral HSV-2 outbreaks tend to recur less frequently than oral HSV-1.
What a Cold Sore Looks and Feels Like
Cold sores progress through five stages and generally last 7 to 12 days from first symptom to full healing.
The first sign is a tingling, itching, or burning sensation on or near the lip, often in the same spot as previous outbreaks. This prodrome stage lasts several hours to two days. The area may swell and redden before any blister appears. Within about 48 hours, one or more fluid-filled blisters form. These eventually rupture, leaving a raw, weeping area that is painful and highly contagious, as the fluid is packed with active virus. After roughly three days, a scab forms. The scabbing phase lasts two to three days and often involves cracking, minor bleeding, and itching. Finally, the scab falls away and the skin heals, usually without scarring.
Cold sores are contagious at every stage, but the weeping blister phase carries the highest risk of transmission.
Treatment Options
Antiviral medications can shorten an outbreak and reduce its severity, but they work best when started at the very first tingling sensation. The most commonly prescribed oral antiviral for cold sores is taken as two doses in a single day, 12 hours apart. Over-the-counter topical creams containing antiviral agents can also help, though they’re generally less effective than oral medication. For people who get frequent outbreaks (six or more per year), daily suppressive antiviral therapy can reduce the number of recurrences significantly.
Beyond medication, keeping lips protected from sun with SPF lip balm can prevent UV-triggered outbreaks. Avoiding direct contact with others during an active sore, washing hands frequently, and not sharing items that touch the mouth all reduce the chance of passing the virus along.
Complications Worth Knowing About
For most people, cold sores are a nuisance rather than a serious health threat. But the virus can spread to other parts of the body, sometimes with more significant consequences.
Touching a cold sore and then rubbing your eye can transfer the virus to the cornea, causing a condition called herpes keratitis. Symptoms include eye pain, redness, blurred vision, light sensitivity, and watery discharge. Herpes keratitis is a leading cause of blindness worldwide, though most cases heal without permanent damage when treated promptly. The same hand-to-eye transfer can be prevented by thorough handwashing during an active outbreak.
The virus can also infect the fingers (herpetic whitlow), producing painful blisters on the fingertips or around the nails. This is most common in people who touch their cold sores frequently or in healthcare workers exposed to oral secretions. In people with weakened immune systems, such as those undergoing chemotherapy or organ transplant recipients, HSV-1 reactivation can be more severe and widespread, sometimes requiring longer courses of antiviral treatment.