Herpes simplex type 1 is a herpes virus that causes cold sores and fever blisters in and around the mouth.
In rare cases, 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 herpes simplex virus type 1 (HSV-1) human herpesvirus 1 (HHV-1).
From 
Skin Problems and Treatments Resources
References
Jameson, J. Larry, et al. Harrison's Principles of Internal Medicine, 20th Ed. New York: McGraw-Hill Education, 2018.