Identify which pathogen is the least virulent on the basis of ID50:
pathogen A with an ID50 of 1000 cells
The ID50 (infectious dose 50) represents the number of microorganisms required to infect 50% of a population exposed to the pathogen. Therefore, the higher the ID50, the less virulent the pathogen is.
As per this definition, the pathogen with the highest ID50 would be the least virulent. For example, if Pathogen A has an ID50 of 10^8 (meaning that 50% of the population exposed to the pathogen will be infected when they are exposed to 10^8 bacterial cells), while Pathogen B has an ID50 of 10^2 (meaning that 50% of the population exposed to the pathogen will be infected when they are exposed to 100 bacterial cells), Pathogen A is less virulent than Pathogen B because it requires a larger dose to infect 50% of the population.
Therefore, the pathogen with the least virulence would have the highest ID50.
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