Flood Swarms are a new mother hugger of the Flood army of horror that only appear in Fail-o Wars. It is theorized they are Drones infected with AIDS. They are bird monsters with an attitude like they have bird flu and try to give Gorillas and Marines the flu by shitting on them. To kill vehicles, Swarms drop their white shit on the windshield with the hope that so much crap will cause the vehicle to lose control and careen off a cliff to its destruction. These ways only work on Legendary, everything else is a 0% chance.
Swarms are very fragile things that can be killed by a good pistol blast or a good old fashioned burn. Jackholes and Spartans like to use Swarms as hunting targets in training or at a duck hunting lake. After ten years of being killed, the Swarm population dropped to a damn number in just the double digits. This is also the result that the Flood found out their infected Drones have no boy-girl parts and its hard to jump up to a drone to give it AIDS. Dr. Phil, in particular, was mad and left the Swarms at the Dreadnought world like they were his unwanted kids. The Flood no longer wanted anything to do with the Swarms since they couldn't kill worth a damn.
When the smelly Humans arrived at the Dreadnought world to kill the Arbiter, the Swarms found themselves, yet again, screwed. Failing to kill you, the Swarms ran off to the woods to hide. When Warthog Guy detonated a Chernobyl reactor in the sun, the Swarms were all burned away into nothing but fried chicken. They tasted much better than fried Jackal! No Swarms were left to later annoy the Chief in Halo: Combat Evolved, Halo 2, and Halo 3.