Are part of the body's immune system and patrol the body for diseased cells like virus-infected and cancer cells.

The answer is natural killer cell (NK).

The NK cell ("Natural Killer" cell) is part of the lymphocytes because it is derived from the lymphoid progenitor in the bone marrow.The NK cell can kill target cells spontaneously, using MHC class 1 molecules, and is able to differentiate between a healthy cell and a "sick" cell, like cell infected with a virus or a tumor cell.