Cold sore

A cold sore is a contagious viral infection of the lips and surrounding face, caused by the herpes simplex virus and marked by small, painful, fluid-filled blisters. The infection is common and usually caught in childhood through close contact such as kissing; the virus then stays dormant in facial nerves for life and can reactivate. Recurrences may follow illness, fever, sunlight, stress or hormonal change such as menstruation. Episodes begin with tingling, itching or burning before blisters burst and crust over; a first infection can also cause fever, sore throat and swollen glands. Infection near the eye, in newborn babies or in people with weakened immunity can be serious. Diagnosis is usually clinical, and a sample from the sore can undergo viral culture or DNA testing. Treatment uses antiviral creams, which work best before blisters form, plus pain relief, with antiviral tablets for large, painful or recurring sores.

Seek urgent care for a high fever with a stiff neck, rash, confusion, difficulty breathing, or fever after travel to a malaria area. See a doctor when a fever lasts more than three days, when symptoms return after treatment, or when you may have been exposed to an infection that needs testing. Call the local emergency number for any symptom that is sudden and severe. This page is a reference for healthcare professionals and does not replace an assessment.