What is agent (of disease)?
agent (of disease) is A factor, such as a micro-organism, chemical substance, form of radiation, or excessive cold or heat, which is essential for the occurrence of a disease. A disease may be caused by more than one agent acting together or, in the case of deficiency diseases, by the absence of an agent.
source: A GLOSSARY OF TERMS FOR COMMUNITY HEALTH CARE AND SERVICES FOR OLDER PERSONS. World Health Organization, 2004.