To be infected by the flood, an organism needs to have enough biomass and calcium in its body. Hunters can't be infected because they lack a central nervous system for the flood to take control of, a skeleton, and the individual worm that make up the Hunters are too small to be infected. Drones don't have any bones, they have an exoskeleton, so they don't have enough calcium for the flood to use in them. Grunts and Jackals can be infected, but they usually end up as carrier forms, like Twinreaper said. There are grunt and jackal combat forms in Halo Wars, though they are never seen in any other games. Any creature with biomass for food or the creation of a Gravemind.