I'm not speaking as an authortiative individual here, but likely it's defined probabilistically in that 50% of people given a dosage show signs of infection lat

I'm not speaking as an authortiative individual here, but likely it's defined probabilistically in that 50% of people given a dosage show signs of infection later. See more here: https://en.wikipedia.org/wiki/Minimal_infective_dose https://en.wikipedia.org/wiki/Minimal_infective_dose