AGENT

An active and efficient cause; capable of producing a certain effect

Example: "Their research uncovered new disease agents"

A representative who acts on behalf of other persons or organizations

A substance that exerts some force or effect

A businessman who buys or sells for another in exchange for a commission

Any agent or representative of a federal agency or bureau

The semantic role of the animate entity that instigates or causes the happening denoted by the verb in the clause