Cold sores are caused by herpes simplex virus type 1, commonly called HSV-1. An estimated 3.8 billion people under age 50 carry this virus, making it one of the most widespread infections on the planet. Most people pick it up during childhood through ordinary contact like a kiss from a parent or sharing a cup.
HSV-1 vs. HSV-2
There are two types of herpes simplex virus. HSV-1 is the one responsible for the vast majority of oral herpes, producing the blisters on or around the lips that people call cold sores or fever blisters. HSV-2 is the type more commonly associated with genital herpes. That said, either type can infect either location. HSV-1 can cause genital herpes through oral sex, and HSV-2 occasionally causes oral sores, though this is far less common.
The key difference for most people is simply where each type prefers to set up shop. HSV-1 favors the nerve cluster near the jaw (the trigeminal ganglion), which supplies sensation to the face and lips. HSV-2 typically settles into the nerve cluster at the base of the spine, which serves the genital area. This preference for specific nerve pathways is what drives the pattern of where outbreaks appear.
How the Virus Stays in Your Body
Once you’re infected with HSV-1, it never leaves. After the initial outbreak heals, the virus travels along nerve fibers and parks itself inside nerve cells near the base of the skull. There it enters a dormant state, essentially hiding from the immune system by going quiet. The virus can sit silently for weeks, months, or even years before reactivating and traveling back down the nerve to the skin surface, producing a new outbreak.
Not everyone who carries HSV-1 gets cold sores. Many people are completely asymptomatic, meaning they never develop visible blisters. But even without symptoms, the virus can still be present on the skin surface. Research shows that at least 70% of people carrying HSV-1 shed the virus from their mouth at least once a month without any visible sores. On any given day, about one in three carriers has detectable virus in their saliva. These shedding episodes are brief, typically lasting one to three days, but the virus is present in quantities sufficient to spread to someone else.
What Triggers an Outbreak
Several well-documented triggers can wake the virus from dormancy. Sunlight exposure is one of the most common, which is why some people reliably get cold sores after a day at the beach or on the ski slopes. Psychological stress activates the body’s stress hormone pathways, and those same hormones can stimulate the virus to reactivate. Fever and illness are classic triggers, so much so that “fever blister” became an alternate name for cold sores. Other known triggers include menstruation, fatigue, and physical trauma to the face or lips.
Some people experience frequent outbreaks while others go years between them. The reasons for this variation aren’t fully understood, but individual differences in immune response play a major role.
What a Cold Sore Looks and Feels Like
A cold sore outbreak typically follows a predictable sequence over 5 to 10 days. It starts with a tingling, itching, or burning sensation on the lip, often before anything is visible. This is the prodrome stage, and it’s the best window for starting treatment. Within a day or two, small fluid-filled blisters cluster together on or near the lip border. These blisters eventually rupture into shallow open sores, which then crust over and gradually heal.
The blisters appear on the outside of the mouth, usually along the border where the lip meets the skin. This is an important distinction from canker sores, which are a completely different condition. Canker sores show up inside the mouth as single round white or yellow sores with a red border. They are not caused by herpes and are not contagious. Cold sores, by contrast, are clusters of small fluid-filled blisters on the outer lip, and they are highly contagious from the tingling stage until the sore is fully healed.
How Cold Sores Spread
HSV-1 spreads primarily through direct contact with an active sore or with saliva from someone shedding the virus. Kissing is the most common route of transmission. Sharing utensils, lip balm, razors, or towels can also transmit the virus, though direct skin-to-skin contact is far more efficient. The virus is most contagious when blisters are open and weeping, but transmission during asymptomatic shedding accounts for a significant share of new infections.
You can also spread HSV-1 to other parts of your own body. Touching a cold sore and then rubbing your eye can cause herpes keratitis, an eye infection that leads to pain, redness, blurred vision, and light sensitivity. Left untreated, it can scar the cornea and potentially cause vision loss. Washing your hands after touching a cold sore is one of the simplest ways to prevent this.
Treatment Options
Antiviral medications can shorten outbreaks and reduce their severity, but they work best when started early, ideally during that initial tingling stage. For cold sores specifically, treatment is typically a short course, sometimes as brief as a single day. People with frequent outbreaks may take a lower daily dose continuously to suppress the virus and reduce how often sores appear.
Over-the-counter creams containing the antiviral docosanol can modestly reduce healing time if applied at the first sign of tingling. Cold compresses and pain relievers help manage discomfort during an active outbreak. Keeping the area clean and dry speeds healing and reduces the risk of bacterial infection on top of the sore.
For people who notice their outbreaks follow a pattern, like appearing after sun exposure, preventive steps can help. Using lip balm with SPF, managing stress, and getting adequate sleep won’t eliminate the virus, but they can reduce the frequency of flare-ups over time.
How Common HSV-1 Really Is
Roughly 64% of the global population under 50 carries HSV-1, according to the World Health Organization. In many regions, the rate is even higher. Most people acquire the virus in childhood, often before age 5, through casual contact with family members. Because the majority of carriers never develop noticeable symptoms, most people who have HSV-1 don’t know it. The social stigma around herpes is disproportionate to how ordinary and widespread this infection actually is.