What causes a fever blister without having a fever?

A fever blister, also known as a cold sore, is primarily caused by the herpes simplex virus (HSV), particularly HSV-1. It can occur without a fever because the virus can be triggered by various factors such as stress, sun exposure, hormonal changes, or a weakened immune system, leading to the outbreak of sores around the lips or mouth. The name "fever blister" is somewhat misleading, as a fever is not always present during an outbreak.