Picture of Herpes Simplex Virus Type 1

Herpes simplex type 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 herpes simplex 1:

In rare cases, as when someone's immune system is severely compromised, this virus can cause infection of the brain (encephalitis), requiring intravenous therapy.

Herpes simplex type 1 is also known as human herpesvirus 1 (HHV-1).