Cold sores are caused by herpes simplex virus type 1 (HSV-1), a virus that infects roughly 3.8 billion people under age 50, about 64% of the global population. What makes cold sores so persistent isn’t the blister itself but what happens between outbreaks: the virus hides inside your nerve cells for life, reactivating periodically to produce new sores.
How the Virus Gets In and Stays Forever
HSV-1 enters your body by infecting the outer layer of skin or mucous membrane, typically around the mouth. During this initial infection, the virus replicates aggressively, producing copies of itself that spread to surrounding cells. But the virus also does something more strategic: it finds the endings of nearby sensory nerves and slips inside them.
Once inside a nerve fiber, the virus travels backward along the nerve’s long extension (the axon) toward the nerve cell body. For oral cold sores, the destination is a cluster of nerve cells called the trigeminal ganglion, located near the base of your skull. This journey from the skin surface to the nerve cluster is called retrograde axonal transport, and it’s the key move that makes the infection permanent.
Inside those neurons, the virus essentially shuts itself down. Its DNA forms a circular loop and wraps around proteins called histones, mimicking the way your own genes are packaged. Nearly all of the virus’s genes go silent. The only thing still being produced is a set of non-coding molecules called latency-associated transcripts, which appear to help the virus survive in this dormant state without being destroyed by the cell. Your immune system can’t clear a virus that isn’t actively making viral proteins, so the infection persists for life.
What Triggers a Cold Sore to Reappear
Reactivation happens when something disrupts the balance keeping the virus dormant. A variety of stimuli can flip the switch, activating viral genes that had been silent and kicking off production of new infectious virus particles. Common triggers include:
- UV exposure: Sunlight on the lips is one of the most reliable triggers. If sun is a known trigger for you, sunscreen on and around the lips can help prevent outbreaks.
- Stress and fatigue: Physical or emotional stress alters immune function and hormone levels, both of which can disturb the virus’s dormant state.
- Illness or fever: A weakened immune system during a cold or flu gives the virus an opening, which is why cold sores earned their name.
- Hormonal shifts: Menstruation is a common trigger for some people.
- Physical trauma to the area: Dental work, facial surgery, or even windburn around the lips can provoke reactivation.
The biological details vary by trigger, but the end result is the same: viral genes that were silenced begin expressing again, the neuron starts producing new virus particles, and those particles travel back down the nerve fiber to the skin surface. The virus then infects skin cells around the lip, replicates rapidly, and produces the visible sore.
The Stages of a Cold Sore, Day by Day
A cold sore follows a predictable timeline from the first sensation to full healing, typically lasting about two weeks.
On day 1, you feel a tingling, itching, or numbness on or near your lip. This is the prodromal stage, when the virus has reached the skin but hasn’t yet caused visible damage. It’s the most important window for treatment, because antiviral medication works best before blisters form.
Within 24 hours, small bumps appear and quickly fill with clear fluid, forming the characteristic blisters. By days 2 to 3, those blisters rupture and ooze clear or slightly yellow fluid. This weeping phase is when the sore is most contagious, because the fluid is packed with active virus. By days 3 to 4, the oozing stops and a crust or scab forms over the sore. The scab typically falls off on its own within 6 to 14 days after the outbreak started.
How Cold Sores Spread
HSV-1 spreads through direct contact with an active sore or with the fluid it releases. Kissing is the most common route, but sharing utensils, lip balm, razors, or towels can also transfer the virus. The risk is highest during the weeping stage when blisters have ruptured.
What makes HSV-1 especially difficult to contain is that the virus can also shed from the skin surface without any visible sore present. This is called asymptomatic shedding, and it means someone can transmit the virus even when they feel perfectly fine and have no blisters. Shedding episodes are typically brief and involve smaller amounts of virus than an active outbreak, but they account for a meaningful share of new infections precisely because people don’t know to take precautions.
Cold Sores vs. Canker Sores
These two get confused constantly, but they’re entirely different conditions. Cold sores (fever blisters) appear outside the mouth, usually along the border of the lips. They show up as clusters of small fluid-filled blisters. Canker sores appear inside the mouth and are single round sores, white or yellow with a red border. Canker sores are not caused by a virus and are not contagious.
Managing Outbreaks
Antiviral medications are the standard treatment. They work by blocking the virus’s ability to copy its DNA, slowing replication and giving your immune system time to contain the outbreak. Starting treatment during the tingling stage, before blisters appear, produces the best results. Taken early, antivirals can shorten healing time and reduce the severity of the sore. For people who get frequent outbreaks (six or more per year), daily suppressive antiviral therapy can reduce how often sores appear.
Over-the-counter options include topical creams that contain a skin-penetrating antiviral agent. These offer modest benefit if applied at the first sign of tingling. Pain relief creams or patches that numb the area can make the outbreak more comfortable but don’t affect healing speed.
Preventing Outbreaks
Because triggers vary from person to person, prevention starts with identifying your pattern. If your cold sores tend to follow sun exposure, applying sunscreen or a lip balm with SPF before going outside is one of the simplest and best-supported preventive steps. If stress is your trigger, sleep, exercise, and stress management aren’t just general wellness advice; they directly reduce your outbreak frequency.
You may have seen lysine supplements marketed for cold sore prevention. The amino acid has a plausible mechanism (it competes with another amino acid the virus needs to replicate), but the clinical evidence supporting it is weak. Harvard Health Publishing notes that the evidence for lysine preventing cold sores “is not very good.” It’s unlikely to cause harm at typical supplement doses, but it shouldn’t be relied on as a primary strategy.
The most effective prevention for people with frequent outbreaks remains daily antiviral therapy, which can reduce both the number of outbreaks and the likelihood of transmitting the virus to others.