LD50 and ID50 are both measures of a pathogen’s virulence. However, as these are normally studied in murine models, they can ignore differences found in humans.
These measures are cumulative, taking into account all steps in an infection. This may skip over differences between the model organism being used and humans.
It is possible to compare closely related strains of pathogens with IC50/LD50 (isogenic mutants), but different pathogens cannot be compared with these measures.