Cold sores are caused by herpes simplex virus type 1 (HSV-1), and you get it through direct contact with an infected person’s skin, saliva, or sores. Most people pick up the virus during childhood or young adulthood from nonsexual contact, often from a kiss by a parent or relative. Once you have it, the virus stays in your body permanently, hiding in nerve cells and reactivating periodically to produce new sores.
How the Virus Spreads
HSV-1 spreads primarily through skin-to-skin contact. The most common routes are kissing someone who has the virus and coming into contact with their saliva. You can catch it from someone with a visible cold sore, but also from someone with no symptoms at all. The virus can be present in saliva and on skin surfaces in and around the mouth even when no sore is visible.
Oral sex is another transmission route. HSV-1 can spread from the mouth to the genitals or from the genitals to the mouth during oral contact. This means cold sores aren’t exclusively an “oral” problem, and genital herpes isn’t always caused by HSV-2.
One common worry is catching cold sores from objects like cups, utensils, or towels. The CDC notes that you will not get herpes from touching objects such as silverware, soap, or towels. While HSV can technically survive on dry surfaces for anywhere from a few hours to several weeks (longer in low-humidity environments), real-world transmission from objects is not considered a meaningful risk. The virus needs to reach your skin or mucous membranes in sufficient quantity, which casual surface contact rarely provides. That said, sharing items that directly touch saliva, like a lip balm applied to an active sore, is still worth avoiding.
Asymptomatic Shedding
One of the trickiest things about HSV-1 is that people can spread it without knowing they have it. This is called asymptomatic shedding, meaning the virus is active on the skin surface but causes no visible sore or symptoms. Research from the University of Washington tracked how often people shed HSV-1 and found that in most instances of shedding, participants had no symptoms at all.
In the first few months after infection, participants shed the virus on about 12% of days. By 11 months, that dropped to 7% of days. Among those who still shed frequently at 11 months, the rate fell further to just 1.3% of days by the two-year mark. For comparison, HSV-2 shedding is far more persistent, occurring on roughly 34% of days in the first year and 17% of days a decade later. So while asymptomatic shedding of oral HSV-1 does happen, it becomes less frequent over time.
What Happens After You’re Infected
When HSV-1 first enters your body, it infects skin or mucous membrane cells by attaching to their surface and fusing with the cell membrane. The virus replicates in those cells, which is what produces the initial sore. But the real reason cold sores keep coming back is what happens next.
After the first infection clears, the virus travels along nerve fibers and settles into a cluster of nerve cells called the trigeminal ganglion, located near your jaw. There it goes dormant, essentially hiding inside neurons without producing any virus. Your immune system can’t reach it in this state, which is why the infection is lifelong. The virus isn’t actively doing anything during this dormant phase, and you won’t have symptoms.
What Triggers a Cold Sore to Come Back
Periodically, something wakes the dormant virus up. It travels back down the nerve fibers to the skin surface, begins replicating again, and produces a new sore. Researchers at the University of Virginia identified the core mechanism: stimuli that cause inflammation or overexcite the nerve cells housing the virus can trigger reactivation. The known triggers include:
- Stress: Prolonged emotional or physical stress creates inflammatory conditions that can reactivate the virus.
- Sunburn and UV exposure: Ultraviolet light damages skin cells around the lips, releasing inflammatory signals that travel to the nerve cells where the virus is hiding.
- Fever and illness: Being sick puts your immune system under strain, which is why cold sores often appear during or after a cold or flu (and why they’re called “fever blisters”).
- Fatigue and sleep deprivation: Both weaken immune surveillance, giving the virus a window to reactivate.
- Hormonal changes: Menstruation is a common trigger for some people.
Not everyone who carries HSV-1 gets frequent cold sores. Some people have one outbreak and never another. Others get several per year. The frequency tends to decrease as you get older and your body builds a stronger immune memory against the virus.
How Common Cold Sores Really Are
HSV-1 is one of the most widespread infections on the planet. The World Health Organization estimates that roughly two-thirds of the global population under age 50 carries HSV-1. Most people acquire it during childhood from everyday nonsexual contact, like a parent’s kiss on the cheek or sharing food. Many never develop a visible cold sore and have no idea they carry the virus.
Because the majority of transmission happens in childhood through normal family contact, cold sores are not primarily a sexually transmitted infection, though sexual transmission is possible. The sheer prevalence of HSV-1 means that avoiding it entirely is difficult, and having it is far more common than not.
Reducing Your Risk of Catching or Spreading It
If someone near you has an active cold sore, avoid kissing them or sharing items that have touched their saliva, like drinks, utensils, or lip products. The virus is most contagious when a sore is present and weeping fluid, though it can spread at other times too.
If you already carry HSV-1, you can reduce the chance of spreading it by avoiding close contact with others during an outbreak, especially with newborns or anyone with a weakened immune system, for whom HSV-1 can be more dangerous. Keeping your hands away from an active sore and washing them frequently during an outbreak also helps, since touching a sore and then touching your eyes, for example, can spread the virus to a new location on your own body.
For people who get frequent outbreaks, antiviral medications can shorten the duration of a sore and reduce how often they occur. These work best when taken at the first sign of a tingling or burning sensation, which typically appears a day or two before the sore itself.