A cold sore usually passes through several stages:

• Tingling and itching. Many people feel an itching, burning or tingling sensation around their lips for a day or so before a small, hard, painful spot appears and blisters erupt.
• Blisters. Small fluid-filled blisters typically break out along the border where the outside edge of the lips meets the skin of the face. Cold sores can also occur around the nose or on the cheeks.
• Oozing and crusting. The small blisters may merge and then burst, leaving shallow open sores that will ooze fluid and then crust over.

Signs and symptoms vary, depending on whether this is your first outbreak or a recurrence. They can last several days, and the blisters can take two to four weeks to heal completely. Recurrences typically appear at the same spot each time and tend to be less severe than the first outbreak.

During first-time outbreaks, some people also experience:

• Fever
• Painful eroded gums
• Sore throat
• Headache
• Muscle aches
• Swollen lymph nodes

Children under 5 years old may have cold sores inside their mouths and the lesions are commonly mistaken for canker sores. Canker sores involve only the mucous membrane and aren't caused by the herpes simplex virus.

Cold sores are caused by certain strains of the herpes simplex virus (HSV). HSV-1 usually causes cold sores. HSV-2 is usually responsible for genital herpes. However, either type can cause sores in the facial area or on the genitals. Most people who are infected with the virus that causes cold sores never develop signs and symptoms.

Cold sores are most contagious when oozing blisters are present. But you can transmit the virus to others even if you don't have blisters. Shared eating utensils, razors and towels, as well as kissing, may spread HSV-1. Oral sex can spread HSV-1 to the genitals and HSV-2 to the lips.

Once you've had an episode of herpes infection, the virus lies dormant in nerve cells in your skin and may emerge as another cold sore at the same place as before. Recurrence may be triggered by:

• Viral infection or fever
• Hormonal changes, such as those related to menstruation
• Stress
• Fatigue
• Exposure to sunlight and wind
• Changes in the immune system

About 90 percent of adults worldwide — even those who've never had symptoms of an infection — test positive for evidence of the virus that causes cold sores.

People who have weakened immune systems are at higher risk of complications from the virus. Medical conditions and treatments that increase your risk of complications include:

• HIV/AIDS
• Severe burns
• Eczema
• Cancer chemotherapy
• Anti-rejection drugs for organ transplants

In some people, the virus that causes cold sores can cause problems in other areas of the body, including:

• Fingertips. Both HSV-1 and HSV-2 can be spread to the fingers. This type of infection is often referred to as herpes whitlow. Children who suck their thumbs may transfer the infection from their mouths to their thumbs.
• Eyes. The virus can sometimes cause eye infection. Repeated infections can cause scarring and injury, which may lead to vision problems or blindness.
• Widespread areas of skin. People who have a skin condition called eczema are at higher risk of cold sores spreading all across their bodies. This can become a medical emergency.
• Other organs. In people with weakened immune systems, the virus can also affect organs such as the spinal cord and brain.