I'm a bit confused by the concept of a minimum infectious dose (MID). It seems from what my research so far has turned up, that while any dose greater than 0 could potentially cause an infection, for most pathogens the likelihood of this occurring is low enough to be dismissed.
What I'm now trying to figure out is, what metric is used to define the MID? Is it the amount of the pathogen which is expected to cause an infection in 50% of those exposed (similar to an LD50?) Is there some other value used instead?
Secondly, is there a specified relationship between the received dose, as a multiple of the MID, and the likelihood of infection (for example if the MID caused an infection in 50% of people, then perhaps 2 MIDs would cause infection in 75% and 3 MIDs would cause infection in 87.5%).
Note, I am not asking about how the MID is empirically measured, but rather about the conceptual underpinnings of the concept itself. Also, any links you can provide to additional resources would be most appreciated. Google is useless these days now that the search results are flooded with nothing but SARS-CoV-2 specific articles.