Why would immunosuppressant drugs make a patient susceptible to disease?

Weaken immune system: Immunosuppressant drugs are used to weaken overactive immune systems that attack other normal cells in the body. This is called autoimmunity. The primary role of the immune system is to fight infection and to seek out & destroy cancer cells. Immunosuppressant drugs also compromise this vital role of the immune system.

See below.: Immunosuppressive medications turn off the cells that are responsible for fighting infections.