Final answer:
The LD50 and ID50 both refer to the concentration of a pathogen that impacts 50% of a test population; LD50 refers to lethality while ID50 refers to infection. Pathogen A, with an ID50 of 50 particles, is the most virulent pathogen out of the three mentioned as it requires the least number of particles to infect 50% of the population.
Step-by-step explanation:
The concentration of virus that infects or kills 50% of the host cells is referred to as both the LD50 and ID50. The LD50, or median lethal dose, is the concentration of pathogen that kills 50% of infected test animals. The ID50, or median infectious dose, is the concentration of pathogen that will produce active infection in 50% of test animals inoculated. A lower ID50 value indicates a more virulent pathogen, able to infect more hosts with a lesser quantity of pathogenic particles.
For example, among Pathogen A (ID50 of 50 particles), Pathogen B (ID50 of 1,000 particles), and Pathogen C (ID50 of 1 × 106 particles), Pathogen A is the most virulent as it requires the fewest number of particles to infect 50% of the host population.