ABSTRACT
Recall that an agent is a necessary factor for the occurrence of disease and other morbid conditions. An agent can be a microorganism, chemical, energy form, individual/group behavior, or genetic message whose presence or, in some conditions, relative absence (e.g., nutritional deficiency) is needed for disease occurrence. A disease can have more than one agent. Accordingly, disease occurrence also requires the sufficiency factors of host and environment, all interacting within the context of time (Last, 1988).
