Epidemiology of Infectious Diseases: Mode of Transmission (Page 3 of 3) Indirect, whereby an agent is carried from a reservoir to a susceptible host by suspended air particles or by animate (vector) or inanimate (vehicle) intermediaries. Indirect transmission may be accomplished through: vectors such as mosquitoes, fleas, ticks (animate) vehicles such as objects, food, water, milk, or air (inanimate). Airborne transmission involves respiratory droplet nuclei (dried residue of a droplet) and dust. These dried residue remain suspended in air, are infectious for varying lengths of time, and are dangerous because they are of such size that they are quite easily drawn into the lungs and retained. Tuberculosis and respiratory diseases are contracted this way.