Multiplicity of Infection

Multiplicity of infection (MOI) is the ratio of adsorbed, attached, or infecting agents to susceptible targets. This ratio will vary as a function of numbers of infecting agents, how fast those agents attach, how much time is allowed for attachment, and how many targets there are to attach to. Overall, the ratios of infecting agents to targets can be described in terms of Poisson distributions. We discuss three general approaches to MOI determination, arranged in order of increasing precision: estimations based on ratios of agents added to targets (MOI_input), ratios of agents to targets allowing for delays in agent acquisition of targets (MOI_actual), and experimental determinations. The conditions under which MOI_input is applicable are fairly narrow, whereas MOI_actual, while potentially accurate, should be employed more as an approximation rather than providing high accuracy or precision. Experimental determinations, too, can be prone to error. We additionally discuss an MOI-based means of agent-density determination.