ninjatheory:Flood take over the bodies of hosts and assimilate them into Flood. This is done by the Flood Infection Forms. Infection Forms have a long tendrils that pierce the skin of the host and find their way to the spine. They synchronize with the host taking over the brain functions to prevent resistance or cries for help. After this time, the original mind is erased memory by memory, till nothing is left but a "hungry" flood form. Separate tendrils release aggressive Flood DNA into the host, causing the body to mutate into a Combat Form or a Carrier Form. They use the Combat Form for the most obvious reasons, to have a "military" to attack and later capture more able bodies. The only known way to effectively stop a Flood infection is to have Boren's Syndrome, the radiation-caused disease that Sergeant Major Avery J. Johnson has. Boren's does not completely deny the infection, however, which is proven by the fact that Johnson has an increased, but still limited, regeneration ability. Of the many species in Halo, only two are known to produce fully matured Combat Forms; the Humans and the Elites. The Grunts are believed to be mutated into Carrier Forms because they are too small and weak to be effective as Combat Forms. While there has been no evidence as to why the Flood have not shown assimilation of other species, many theories abound. Jackals are believed to have a too porous, bird-like bone structure. Brutes are believed to lack sufficient neural function. Drones are also supposedly ignored for their deficiency of calcium. Some believe that the Flood have certain "tastes", similar to those of Humans. Hunters cannot be successfully assimilated by the Flood due to their bodily makeup consisting of multiple, possibly hundreds of invertebrate entities. It is speculated that other, larger races, such as Drinol become larger Flood organisms, such as Flood Juggernauts, but neither of these creatures has been encountered by humans yet. It is also possible that the Flood has, in fact, assimilated these species, but has yet to exhibit their respective "Forms". The lack of any corpses of any kind after a Flood attack adds even more suspicion as to the real abilities of their assimilation process.
actualy they were going to have flood carrier forms with jackal shields suggesting they can assimilate jackals.