Cold sores are caused by herpes simplex virus type 1 (HSV-1), a highly contagious virus that infects the majority of the world’s population. Once you catch it, the virus stays in your body permanently, hiding in nerve cells near your jawline and reactivating periodically to produce the familiar blisters on or around your lips.
The Virus Behind Cold Sores
There are two types of herpes simplex virus. Type 1 is the primary cause of oral cold sores, while type 2 is more commonly associated with genital herpes. That said, HSV-1 can occasionally cause genital herpes through oral-genital contact, and HSV-2 can sometimes appear on the mouth, though this is less common.
After the initial infection, which may or may not produce noticeable symptoms, HSV-1 travels along nerve fibers and settles into a cluster of nerve cells called the trigeminal ganglion, located near the base of the skull. There, it enters a dormant state. The viral DNA doesn’t merge with your own DNA. Instead, it sits inside the nucleus of the nerve cell as a separate entity, essentially in hibernation. Your immune system can’t reach it or clear it during this phase, which is why the infection is lifelong.
How the Virus Spreads
HSV-1 spreads primarily through direct contact with the virus in sores, saliva, or skin surfaces in and around the mouth. Kissing is the most common route, but sharing utensils, lip balm, razors, or towels can also transmit it. Many people contract HSV-1 in childhood from a parent or close family member.
The virus is most contagious when an active sore is present, but it can also spread when no symptoms are visible. This is called asymptomatic shedding: the virus periodically becomes active on the skin surface without producing a blister you can see or feel. This is one reason HSV-1 is so widespread. In rare cases, a mother with herpes can transmit the virus to her baby during delivery, a serious condition called neonatal herpes.
What Triggers an Outbreak
The virus doesn’t stay dormant forever. Various stressors can wake it up and send it traveling back along the nerve to the skin surface, where it causes a new cold sore. Researchers at the University of Virginia School of Medicine found that when neurons harboring the virus experience “neuronal hyperexcitation,” essentially an overstimulated state, the virus senses the change and reactivates.
Common triggers include:
- Physical stress: fever, illness, fatigue, or a weakened immune system
- Sun exposure: UV light on the lips is a well-known trigger
- Emotional stress: anxiety, sleep deprivation, or major life changes
- Hormonal shifts: menstruation can trigger outbreaks in some people
- Skin trauma: dental procedures, cosmetic treatments, or windburn around the mouth
Some people get cold sores several times a year, while others who carry the virus rarely or never have a visible outbreak. HSV-1 generally causes fewer recurrences over time compared to HSV-2.
The Five Stages of a Cold Sore
A cold sore typically lasts 7 to 12 days and moves through five distinct stages:
- Tingling (Stage 1): A prickling, itching, or burning sensation appears at the site, usually on the lip border. This prodrome stage lasts several hours to 2 days and is the best window for starting treatment.
- Blistering (Stage 2): Within about 48 hours, one or more fluid-filled blisters form, sometimes in a cluster.
- Weeping (Stage 3): The blisters break open, leaving a raw, moist area that is often the most painful phase. This lasts roughly 3 days and is when the sore is most contagious.
- Scabbing (Stage 4): A crust forms over the sore. Cracking, bleeding, and itching are common during this 2- to 3-day period.
- Healing (Stage 5): The scab falls off and the skin heals, usually without scarring.
Managing and Preventing Outbreaks
Antiviral medications can shorten the duration and severity of a cold sore, but they work best when started early, ideally within 72 hours of the first symptoms. Many people learn to recognize that initial tingling and begin treatment right away. Starting antivirals at the prodrome stage can speed up crusting and healing while also reducing how much virus you shed to others.
For people who get frequent or severe outbreaks, daily suppressive antiviral therapy is an option. This approach has been shown to reduce the frequency of recurrences by 70% to 80% and lowers asymptomatic shedding, which means less risk of passing the virus to someone else. Because HSV-1 naturally recurs less often than HSV-2, most people with oral cold sores manage well with episodic treatment rather than daily medication.
Practical steps to reduce outbreaks include wearing SPF lip balm in the sun, managing stress, getting adequate sleep, and avoiding known personal triggers. To avoid spreading the virus during an active sore, skip kissing and sharing items that touch your mouth, and wash your hands frequently, especially before touching your eyes.
Possible Complications
For most people, cold sores are a nuisance rather than a health threat. But the virus can cause more serious problems in certain situations. If HSV-1 reaches the eyes, it can cause herpetic keratitis, an infection of the cornea that may lead to scarring and vision problems. Avoid touching a cold sore and then rubbing your eyes.
People with eczema or atopic dermatitis face a specific risk: the virus can spread across large areas of already-damaged skin, a condition called eczema herpeticum. This can be severe and bilateral if it reaches the eyes, often requiring more aggressive treatment. Individuals with weakened immune systems, including transplant recipients and people with HIV, are also more vulnerable to frequent or complicated outbreaks and are more likely to benefit from ongoing suppressive therapy.