The word “threat” usually stands for a category of things that pose a potential danger. Viruses, worms, and other types of malware, for example, are threats. A “threat agent,” by contrast, is a specific threat, or a specific type of virus, worm, or other malware. For example, the Blaster Worm is a threat agent.
The threat is refereed as as an action which cause constant danger i.e virus and malware and threat agent is refferd to a specific danger,specific virus and specific threat.