An organism capable of causing disease is referred to as what?

The term that describes an organism capable of causing disease is 'pathogen.' This encompasses a variety of microorganisms, including bacteria, viruses, fungi, and parasites, which can invade the body and disrupt normal physiological functions, leading to illness. The pathogenicity of an organism is determined by its ability to overcome the host's immune defenses and cause damage.

Understanding the definitions of the other terms clarifies why 'pathogen' is the most accurate choice. A 'commensal' organism typically lives on or in a host without causing harm, often providing some benefits in a healthy microbiome context. An 'antigen' refers to a substance that the immune system recognizes as foreign, often leading to an immune response, but it does not itself cause disease. An 'allergen' is a type of antigen that triggers an allergic reaction but is not necessarily capable of causing disease in the traditional sense. Therefore, 'pathogen' is the definitive term used in medical contexts for organisms that cause disease.