Halo3 what is with the new flood

In Halo 3, new and strange forms of the Flood are unleashed to make Chief's task that much harder. There are four forms of Flood, which include; Infection Form, the base unit of the Flood scourge, Carrier Form, slow moving "sacks" that carrier infection forms, Combat From, which is the most common and appears to be a mutated marine or elite. Next comes the Pure Forms which are the Stalker, Tank, and the Ranged. The Stalker Form is the least dangerous Pure Form body type. The Tank Form is enormously strong, and carries out savage melee attacks at close range and appears to be a mutated Hunter. The Ranged Form fires incessant volleys of Carbine-like projectiles over long distances. They are stationary, and often cause immense frustration on harder difficulties.