Cold sores form when herpes simplex virus type 1 (HSV-1) reactivates from its hiding place in a nerve cluster near your jaw, travels down nerve fibers to the skin around your lips, and invades the surface skin cells there. The virus forces those cells to produce copies of itself, eventually destroying them and creating the fluid-filled blisters you see on or near your lips. Around 3.8 billion people under age 50, roughly 64% of the global population, carry HSV-1.
Where the Virus Hides Between Outbreaks
After your first exposure to HSV-1, the virus doesn’t leave your body. It travels up nerve fibers and settles into the trigeminal ganglion, a bundle of sensory nerve cells located near your temple, just in front of your ear. Once there, the viral DNA goes quiet. It stops producing the proteins it needs to replicate and instead produces only a single type of RNA molecule associated with latency. In this dormant state, the virus can sit undetected by your immune system for months, years, or an entire lifetime without causing symptoms.
The trigeminal nerve splits into three branches that supply sensation to your forehead and eyes, the middle of your face, and your lower jaw. The branches reaching your upper and lower lips are the ones most relevant to cold sores. When the virus reactivates, it follows these nerve fibers back down to the skin surface, which is why cold sores almost always appear in the same general area each time.
What Triggers Reactivation
The virus shifts from dormant to active when something disrupts the balance between viral latency and your immune system’s ability to keep it suppressed. Common triggers include fever, physical or emotional stress, sun exposure, hormonal shifts (like menstruation), fatigue, and illness. A weakened immune response gives the virus a window to begin replicating again inside the nerve cell and start its journey back toward the skin.
How the Virus Damages Skin Cells
Once the reactivated virus reaches the skin, it enters the outermost layer of cells, called keratinocytes. It attaches to the cell surface using several receptor proteins, then fuses with the cell membrane to get inside. From there, the virus hijacks the cell’s machinery to churn out copies of itself.
As viral copies accumulate, they spread to neighboring cells in two ways: newly assembled virus particles burst out and infect adjacent cells, or the virus triggers direct cell-to-cell fusion, merging an infected cell’s membrane with its neighbor’s. This fusion creates abnormally large, multinucleated cells. The rapid spread destroys clusters of skin cells, a process called “ballooning degeneration,” where cells swell and break apart. The damage creates a pocket within the outer skin layer that fills with fluid, forming the visible blister.
The Five Stages of a Cold Sore
A cold sore follows a predictable timeline from start to finish, typically resolving within 6 to 14 days.
- Tingling stage (day 1): You feel itching, tingling, pain, or numbness on your lip or the skin nearby. No blister is visible yet, but the virus is already replicating beneath the surface.
- Blister formation (days 1 to 2): Within about 24 hours of that first tingle, small bumps appear, most often along the outer edge of your lips. Within hours, these bumps fill with clear fluid and become true blisters.
- Weeping stage (days 2 to 3): The blisters rupture and ooze a clear or slightly yellow fluid. This is the most contagious phase, since the fluid is packed with active virus.
- Crusting stage (days 3 to 4): The oozing stops and a golden-brown crust forms over the sore.
- Healing (days 4 to 14): The scab gradually shrinks and eventually falls off, revealing healed skin underneath. Some redness may linger for a few more days.
When Cold Sores Are Contagious
Cold sores are most contagious during the weeping stage, when open blisters are actively releasing fluid full of virus. But HSV-1 can also spread before a blister appears and after it has crusted over, because the virus sheds from the skin surface even when no sore is visible. This “asymptomatic shedding” is most frequent in the first few months after an initial infection, occurring on roughly 12% of days at two months. By 11 months, that rate drops to about 7% of days, and it continues to decline over years.
Direct skin-to-skin contact is the primary route of transmission. Kissing, sharing utensils or lip products, and touching a sore then touching another person can all pass the virus along. Avoiding contact with the affected area from the first tingle through complete healing significantly reduces the risk of spreading it.
Why Some People Get Frequent Outbreaks
Most people who carry HSV-1 experience outbreaks infrequently or never at all. For those who do get recurring cold sores, the frequency varies widely. Some people have one outbreak a year, others several. The difference comes down to how effectively the immune system keeps the virus suppressed in the nerve ganglion. People who are immunocompromised, under chronic stress, or frequently exposed to known triggers like intense sunlight tend to experience more frequent reactivation. Over time, outbreaks generally become less frequent and less severe as the body builds a stronger immune memory against the virus.
Antiviral medications taken at the first sign of tingling can shorten the duration and severity of an outbreak. For people with frequent recurrences, daily antiviral therapy can reduce the number of outbreaks per year and lower the rate of viral shedding between them.