What Causes Cold Sores? HSV-1 Triggers and Prevention

Cold sores are small, painful blisters that appear on or around your lips. If you've experienced one, you're not alone. About 64% of people worldwide under age 50 carry the virus that causes them. Understanding what triggers these outbreaks can help you reduce their frequency and take control.

The Root Cause: Herpes Simplex Virus Type 1

Cold sores are caused by the herpes simplex virus type 1 (HSV-1). This virus spreads through direct contact with an infected person's saliva, skin, or the fluid from their cold sores. Most people contract HSV-1 during childhood, often from a simple kiss from a family member or by sharing utensils, towels, or lip balm.

Once the virus enters your body, it stays there for life. It travels to nerve cells near your spine and remains dormant until something triggers it to become active again. This is why cold sores can reappear months or even years after your first outbreak.

Common Triggers That Reactivate the Virus

While the virus causes cold sores, specific triggers wake it up from its dormant state. Knowing your triggers is the first step to preventing outbreaks.

Stress and Fatigue

Both physical and emotional stress are among the most common cold sore triggers. When you're stressed or exhausted, your immune system weakens, giving the virus an opportunity to reactivate. Lack of sleep can also trigger outbreaks.

Sun Exposure

Ultraviolet (UV) light from the sun is a well-documented trigger. Sunburn on or around your lips can prompt the virus to become active. Many people notice cold sores appearing after a day at the beach or skiing.

Illness and Fever

When you're fighting off a cold, flu, or other infection, your immune system is busy elsewhere. Research shows that people with febrile (fever-causing) conditions experience cold sores three times more often than those without fever. This is why cold sores are sometimes called "fever blisters."

Hormonal Changes

Hormonal fluctuations during menstruation or pregnancy can trigger outbreaks in some people. These hormonal shifts may temporarily weaken your immune response.

Other Triggers

Additional triggers include lip injuries, dental procedures, extreme weather (hot or cold), and damaged skin from conditions like eczema. Interestingly, new research from 2025 suggests that even detecting another infection in your body may trigger the herpes virus to activate, even if that new infection doesn't fully develop.

Why Some People Get Cold Sores More Often

Not everyone infected with HSV-1 experiences frequent outbreaks. Some people never have symptoms despite carrying the virus, while others deal with monthly flare-ups. Research has identified genetic factors that influence cold sore susceptibility, including specific genes on chromosome 21.

People with weakened immune systems from conditions like HIV, cancer treatments, or immunosuppressive medications tend to experience more frequent and severe outbreaks. On average, people with HSV-1 experience about one outbreak per year, though this varies widely.

How to Minimize Cold Sore Recurrence

While you can't eliminate the virus once you have it, you can reduce how often it activates:

• Protect your lips from the sun with a broad-spectrum lip balm containing SPF 30 or higher
• Manage stress through exercise, adequate sleep, and relaxation techniques
• Maintain a healthy immune system with balanced nutrition and regular physical activity
• Identify and avoid your personal triggers by keeping track of when outbreaks occur
• Consider antiviral medication if you have frequent outbreaks (discuss with your healthcare provider)

When to See a Doctor

Most cold sores heal on their own within 7 to 10 days. However, see a healthcare provider if your cold sores are very painful, last longer than two weeks, occur frequently (more than 6 times per year), or if you have a weakened immune system. You should also seek care if the sores spread to your eyes or other parts of your body.

The Bottom Line

Cold sores are caused by HSV-1, a common virus that remains dormant in your body until triggered by stress, sun exposure, illness, or other factors. While there's no cure, understanding your triggers and taking preventive steps can significantly reduce outbreak frequency. By protecting your lips from the sun, managing stress, and maintaining a strong immune system, you can keep the virus dormant more often and minimize its impact on your daily life.