What is the ID50 referring to for a given pathogen?
O The number of cells or virions required to infect any of the inoculated test population
O The number of cells or virions required to kill 50% of the inoculated test population
O The number of cells or virions required to ingfect 100% of the inoculated test population
O The number of cells or virions required to infect of 50% of the inoculated test population