You get a cold sore by catching herpes simplex virus type 1 (HSV-1), usually through direct contact with an infected person’s saliva, skin, or an active sore around the mouth. Most people pick up the virus during childhood or young adulthood through nonsexual contact, like a kiss from a parent or relative. Once you have HSV-1, the virus stays in your body permanently and can reactivate to produce cold sores throughout your life.
How the Virus Spreads
HSV-1 passes from person to person through contact with the virus in sores, saliva, or the skin in and around the mouth. That means kissing is the most common route, but sharing items that touch saliva, like drinking glasses, utensils, lip balm, or straws, also creates a path for the virus. The CDC notes that most people with oral herpes acquire it during childhood from nonsexual contact with saliva, often before they’re old enough to remember it happening.
HSV-1 can also spread to the genital area through oral sex, causing genital herpes. This is less common but worth knowing about, especially during an active outbreak. You will not, however, get the virus from toilet seats, bedding, swimming pools, or touching objects like soap and towels.
When Cold Sores Are Most Contagious
A cold sore is contagious from the very first tingle or itch until the scab falls off and the skin underneath looks completely normal. That window typically spans about two weeks. But the risk is not equal across all those days.
Cold sores are most infectious within the first 24 hours of forming, when blisters are fresh and the viral load on the skin surface is highest. During days two and three, the blisters rupture and ooze clear or slightly yellow fluid, a stage sometimes called the “weeping phase.” This is still a high-risk period. By days three to four, a golden-brown crust forms, and while the virus can still spread, the risk drops. The scab typically falls off within six to 14 days.
Here’s what surprises many people: you can also spread HSV-1 when you have no visible sore at all. Research shows that at least 70% of people carrying HSV-1 shed the virus from their mouth at least once a month without any symptoms. Many shed it more than six times per month. Each shedding episode typically lasts one to three days. This asymptomatic shedding is why the virus is so widespread and why most people can’t pinpoint when or how they caught it.
What Happens Inside Your Body
When HSV-1 first enters through a break in the skin or a mucous membrane, it infects surface cells and then travels along nerve fibers to a cluster of nerve cells near the base of your skull. There, it essentially goes to sleep. The virus can remain dormant in those nerve cells for months, years, or even an entire lifetime without causing a single sore.
When something triggers reactivation, the virus wakes up and travels back along the same nerve fibers to the skin surface, where it causes a new outbreak. This is why cold sores tend to appear in the same spot each time. The nerve pathway is fixed.
Common Triggers for Outbreaks
Not everyone who carries HSV-1 gets frequent cold sores, and the triggers vary from person to person. The most well-established ones are stress, illness, and sun exposure. Research from UVA Health explains the mechanism: when nerve cells harboring the virus experience heightened stimulation from inflammation or stress signals, the virus senses that change and seizes the opportunity to reactivate.
Prolonged stress or illness prompts your immune system to release inflammatory molecules. Those same molecules are released in skin cells damaged by ultraviolet light, which is why a day of intense sun exposure or a sunburn on the lips can bring on a cold sore within days. Other commonly reported triggers include:
- Fever (which is why cold sores are sometimes called “fever blisters”)
- Fatigue or sleep deprivation
- Hormonal shifts, such as those during menstruation
- Cold, dry, or windy weather that dries and cracks the lips
- Dental work or facial procedures that irritate the area around the mouth
Spreading It to Other Parts of Your Own Body
If you touch an active cold sore and then touch your eye, you can transfer the virus there, potentially causing a serious eye infection. Touching a sore and then putting your fingers near a cut or broken skin elsewhere on your body can also lead to infection in that new location. One well-known example is a painful finger infection called herpetic whitlow, which happens when the virus enters through a break in the skin on the hand. In children, this often results from thumb-sucking during an oral outbreak.
The practical takeaway: wash your hands after touching a cold sore, avoid rubbing your eyes during an outbreak, and if you wear contact lenses, be especially careful about hand hygiene.
Why Some People Get Cold Sores and Others Don’t
A huge portion of the global population carries HSV-1, yet many people never develop a visible cold sore. The difference comes down to individual immune response and genetics. Some people’s immune systems keep the virus suppressed so effectively that it never reactivates, or it reactivates so mildly that no sore forms. Others experience outbreaks several times a year, especially during periods of stress or after sun exposure.
The frequency of outbreaks also tends to change over time. Many people find that cold sores are most frequent in the years shortly after their first infection and become less common as they age. This likely reflects the immune system’s increasing familiarity with the virus over time, though triggers like intense stress or illness can still provoke an outbreak at any age.
The First Outbreak vs. Later Ones
If you do develop symptoms after your initial infection, the first outbreak is usually the worst. It can include multiple sores inside and outside the mouth, swollen gums, sore throat, fever, and swollen lymph nodes. Some people mistake it for a severe case of canker sores or strep throat. This primary outbreak can last two to three weeks.
Recurrent cold sores are typically milder and more predictable. They almost always appear on the outer edge of the lip rather than inside the mouth. The timeline follows a fairly consistent pattern: a tingling or itching sensation on day one, blisters forming within 24 hours, rupturing by day two or three, crusting over by day four, and full healing within about two weeks. Many people learn to recognize the initial tingle as an early warning, which is the best window to start antiviral treatment if you use it.