disease causative agent
the biological pathogen that causes a disease, such as a virus, parasite, fungus, or bacterium
allergen
substance that can produce an abnormally vigorous immune response, in which the immune system reacts to fight off a perceived threat that would otherwise be harmless to the body
pathogen
biological entity that causes disease in its host, which is typically an infectious microorganism or agent, such as a virus, bacterium, protozoan, prion, viroid, or fungus