The cells that are produced in the thymus are the T-lymphocytes or Tcells. The thymus serves as a production and maturation site for T cells.
When a normal cell of the body is invaded and infected by a virus, both the virus and the cell it has invaded have to be killed. The infected cell takes some of fragments of the virus, known as antigen fragments, and displays them on its cell membrane almost like waving a flag. Special T cells called cytotoxic T cells recognize this "flags" and bind to the infected cell. The T cells then release some substances one of which is called perforin, on the infected cell, which poke holes on the cell membrane causing it and the virus within to die.