What is a pathogen?

A pathogen is defined as an organism that causes disease in a host. This includes a variety of biological entities such as bacteria, viruses, fungi, and parasites. The ability of a pathogen to disrupt normal biological functions or provoke an immune response characterizes its pathogenicity, leading to illness or disease. For instance, when a person becomes infected with a pathogen, their body may react by attempting to eliminate the invader, which can often result in symptoms such as inflammation, fever, or other responses that we commonly associate with illness.

The other options do not accurately reflect the definition of a pathogen. A type of immune cell refers to components of the immune system that help combat infections but are not pathogens themselves. A natural body defense mechanism describes processes such as the skin barrier or secretions that protect against pathogens, rather than being a pathogen. Lastly, a non-infectious agent indicates substances or conditions that do not cause infectious diseases, such as toxins or genetic mutations, which also do not fulfill the criteria of being a pathogen.