HHV-1: Human herpesvirus 1. A herpes virus that causes cold sores and fever blisters in and around the mouth. Here is a depiction of a typical fever blister caused by HHV-1:
In rare cases, as when the immune system is severely compromised, this virus can cause infection of the brain (encephalitis). HHV-1 is also known as herpes simplex type (HSV 1) .