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.

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).

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