What term is used for any agent that causes disease?

The term "pathogen" refers to any agent that can cause disease. Pathogens encompass a variety of organisms, including viruses, bacteria, fungi, and parasites, all of which have the potential to lead to illness or infection in a host. This definition makes "pathogen" a broad and inclusive term that is widely used in the fields of medicine and microbiology to describe any infectious agent.

In contrast, while "virus," "bacteria," and even "allergen" can also relate to disease, they are more specific in their definitions. Viruses are a type of pathogen that rely on living hosts to replicate and can cause various diseases, while bacteria are single-celled organisms that can be either beneficial or harmful. Allergens, on the other hand, are substances that can cause allergic reactions, which, although they may lead to disease-like symptoms, do not fit the broader definition of a pathogen as they do not directly cause infections. Thus, "pathogen" is the most accurate term to encompass all agents causing disease.