Multiplicity of infection

In microbiology, the multiplicity of infection or MOI is the ratio of agents (e.g. phage or more generally virus, bacteria) to infection targets (e.g. cell). For example, when referring to a group of cells inoculated with virus particles, the multiplicity of infection or MOI is the ratio of the number of virus particles to the number of target cells present in a defined space.

The actual number of viruses or bacteria that will enter any given cell is a statistical process: some cells may absorb more than one infectious agent while others may not absorb any. The probability that a cell will absorb ${\displaystyle n}$ virus particles or bacteria when inoculated with an MOI of ${\displaystyle m}$ can be calculated for a given population using a Poisson distribution. This application of Poisson's distribution was applied and described by Ellis and Delbrück.

where ${\displaystyle m}$ is the multiplicity of infection or MOI, ${\displaystyle n}$ is the number of infectious agents that enter the infection target, and ${\displaystyle P(n)}$ is the probability that an infection target (a cell) will get infected by ${\displaystyle n}$ infectious agents.

...
Wikipedia