A: Cold sores are a strain of herpes, meaning they are extremely contagious and can recur, says Susan Grant, M.D., an OB-GYN in New York City. They can appear at any time, especially when your body is under stress or going through hormone fluctuations, making pregnancy a prime time for outbreaks.
Cold sores are caused by the herpes simplex virus (HSV).. Both virus types can cause lip and mouth sores and genital herpes.
Cold sores are caused by a virus called herpes simplex. Once you have the virus, it stays in your skin for the rest of your life. Sometimes it causes a cold sore. Most people are exposed to the virus when they're young after close contact with someone who has a cold sore.