Infectious Agent Posted on 1st December 201521st March 2022 by ThePD (The Project Definition) An Infectious Agent is the organisms that causes disease. There are four main classes of infectious agents: bacteria, viruses, fungi, and parasites. Click to share on Twitter (Opens in new window)Click to share on Facebook (Opens in new window)Click to share on LinkedIn (Opens in new window) Related Posted in HSE and tagged health, hse. ThePD (The Project Definition) ThePD has been developing the Preferred Project Definitions based on the actual project execution and operation experiences and knowledge with the Project Language, and sharing with you daily basis.