Human herpesvirus 1 (HHV-1)

Human herpesvirus 1 (HHV-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 .