Identify which pathogen is the least virulent on the basis of $I{D}_{50}$

pathogen A with an $I_{50}$ of $1000$ cells

pathogen $B$ with an $I{D}_{50}$ of $50$ cells

pathogen $C$ with an $I_{50}$ of $1$ cell

virulence cannot be determined from $I{D}_{50}$ values