The flood infection is a parasite that crawls into the dead. Once infected, the corpse is now a "zombie" that fights for the Flood.
Infection Flood are released by the Carrier Forms when they are attacked. These Flood are agile and attack in groups. A single shot from any weapon will effectively kill them, but due to their speed and jumping attack pattern they can be a nuissance as well as a waste of precious ammo. For that reason it is best to temporarily pick up a useless weapon to clear these out if you anticipate or see them in the distance, then re-acquire a more useful weapon.
Do not underestimate these Flood forms. Underestimation of these little beasts are the reason the Flood have spread so effectively. They prey on the unaware and take advantage of the ignorant. They have tiny feelers on the top of their cranium that they use to smell the air to sense any living tissue around. Once they've located an unsuspecting target, they latch on and use tiny teeth to anchor themselves to their prey, they inject their mutagen directly into the prey's bloodstream. It's unclear as to why some victims are turned into Carrier Forms and some are turned int Combat forms.