Infectious or etiologic agent - microorganism (bacteria, fungi, virus, parasites)
Reservoir - source (human beings, animals, inanimate objects, plants, general environment such as air, water, and soil)
Portal of exit from the reservoir - sputum, emesis, stool or blood
Modes of Transmission
Portal of entry - permits the organism to gain entrance into the host. This could be the mucous membrane, non-intact skin, GI tract, GU tract, respiratory tract
Susceptible host - a person who is at risk for infection; whose own body defense mechanism when exposed is unable to withstand the invasions of pathogens