Definition of Herpes simplex type 1

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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).

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