What is a pathogen?

• A. A disease causing organism
• B. A substance that triggers autoimmunity
• C. A non-living agent
• D. A harmless microbe

Answer: (A)

A pathogen is a disease-causing biological agent. It can be a living organism such as bacteria, fungi, or parasites, and it can also include non-living agents like certain viruses that still cause illness in a host. The essence is the ability to produce disease, not just to exist in the environment. A substance that triggers autoimmunity is an immune trigger or antigen, not necessarily something that causes infection. A harmless microbe doesn’t cause disease, so it isn’t a pathogen.