Cold sores are caused by herpes simplex virus type 1 (HSV-1), a highly common infection that spreads primarily through oral contact. Once you’re infected, the virus never leaves your body. It hides in nerve cells near your jaw and can reactivate throughout your life, producing the familiar painful blisters on or around your lips.
The Virus Behind Cold Sores
HSV-1 is the main cause of oral herpes. It spreads through direct contact, most often through kissing, sharing utensils, or skin-to-skin touch with someone who has the virus. Most people pick it up during childhood from a family member, sometimes without ever noticing symptoms.
There are two types of herpes simplex virus. Type 1 primarily causes oral infections (cold sores), while type 2 primarily causes genital infections. HSV-1 can occasionally cause genital herpes too, but oral outbreaks are by far its most common form. The two viruses are closely related but behave differently: HSV-2 is much more likely to cause frequent recurring symptoms than HSV-1 in the genital area.
Why Cold Sores Keep Coming Back
After the initial infection, HSV-1 travels along your nerves and settles into a cluster of nerve cells near your jaw called the trigeminal ganglion. There, it enters a dormant state called latency. The virus essentially goes silent, shutting down nearly all of its activity except for producing a small piece of genetic material that helps it survive inside neurons without being destroyed by your immune system.
Your immune system knows the virus is there. Specialized immune cells patrol the area constantly. But the virus protects the neurons it lives in from self-destructing, a process called apoptosis, keeping both the nerve cell and itself alive. This standoff can last a lifetime, which is why HSV-1 is a permanent infection. When something disrupts the balance, whether it’s a weakened immune system or physical stress on the body, the virus can reactivate, travel back down the nerve to your skin, and produce a new cold sore.
Common Triggers for Outbreaks
Not everyone with HSV-1 gets frequent cold sores. Many people rarely or never have visible outbreaks. For those who do, certain triggers are well established:
- Illness or fever. Any infection that taxes your immune system can prompt an outbreak. Fevers are especially effective at incubating cold sores, which is why they’re sometimes called “fever blisters.”
- Stress. Both emotional and physical stress weaken your immune response. Short-term stress floods your body with cortisol and adrenaline, while long-term stress causes chronic inflammation that diverts immune resources away from keeping HSV-1 in check.
- Sun exposure and extreme temperatures. UV light, sunburns, and very cold weather can all trigger outbreaks. Cold weather also dries and cracks lip skin, creating an additional opening for the virus.
- Hormonal changes. Menstruation, pregnancy, puberty, and menopause can all precede outbreaks in some people.
- Sleep deprivation. Poor sleep weakens immune function and raises susceptibility.
- Lip injuries or cosmetic procedures. Any trauma to the lips, including filler injections, permanent makeup, or even a bruise, can trigger a flare.
- Immunosuppression. Medications that intentionally suppress the immune system (for organ transplants or cancer treatment, for example) increase outbreak frequency.
What a Cold Sore Looks and Feels Like
Cold sores develop and heal over one to two weeks. The first sign is usually a tingling, itching, or burning sensation on or near your lip on day one. Within the next day or two, small fluid-filled blisters appear, often in a cluster. These blisters eventually break open, weep, and then form a golden-brown crust. The scab typically falls off within six to 14 days, sometimes revealing slightly pink or red skin underneath that fades over the following days.
Cold sores are contagious from the very first tingling sensation until the skin has completely healed. This is important to know because many people assume they’re only contagious when blisters are visible.
Spreading Without Symptoms
You don’t need a visible cold sore to pass the virus to someone else. HSV-1 can shed from the skin even when there are no symptoms at all. Research from the University of Washington found that people with HSV-1 shed the virus on about 12% of days in the early months after infection, dropping to around 7% of days by 11 months. In most of those instances, the person had no symptoms while shedding. Over years, shedding rates continued to decline, falling to about 1.3% of days in one group tracked over two years.
This asymptomatic shedding is how many people contract HSV-1 without ever knowing who passed it to them.
Cold Sores vs. Canker Sores
Cold sores and canker sores are completely different conditions that people often confuse. The simplest way to tell them apart is location. Cold sores appear on the outside of the mouth, typically along the border of the lips. Canker sores appear inside the mouth, on the inner cheeks, lips, or tongue.
They also look different. Cold sores are clusters of small, fluid-filled blisters. Canker sores are single, round, white or yellow sores with a red border. Canker sores are not caused by a virus and are not contagious.
Risks for Newborns
HSV-1 poses a serious danger to infants. In newborns, herpes infections can be severe and result in high rates of illness and death. The virus can pass to a baby through direct or indirect contact with active lesions.
If you have a cold sore and are around an infant, thorough hand hygiene is essential. Parents who are breastfeeding can continue as long as there are no lesions on the breast and any sores elsewhere are fully covered. If lesions are present on the breast itself, milk from that side should be discarded until healing is complete, though feeding from the unaffected side is safe.