The guard was "severely beaten by an inmate" sometime Sunday according to the Santa Clara County Sheriff's Department.
The inmate involved in the assault had a history of violence.
Santa Clara County Sheriff Laurie Smith told KCBS that they are dealing with more hardened inmates who used to be housed at state prisons "Because of the prison realignment, where people are serving time locally instead of going to state prison. So, we really need to change how we do business in the custody environment."
The deputy was transported to a nearby hospital to be treated for injuries to his head, and was later discharged.
The deputy was back on the job Monday morning.
Operations at the jail have been under the microscope following the death of Michael Tyree, a mentally ill inmate who was allegedly beaten by guards who are now facing murder charges.
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