Infection Form

Summary
The infection Flood form is nearly harmless at first glance, but once it gains physical contact with a biological host it becomes highly virulent. Small tendrils underneath its "body" penetrate the skin and unleash an attack on the host's nervous system. First, it paralyzes the host's body. Once this is accomplished, it rewrites the neural pathways of the victim's brain with it's tendrils, forcing a resonant frequency match between it's neural signals and the host's. When this is done, it starts to edge its way into the body, slowly moving organs out of the way so it can nest itself into the chest cavity of its host. If a match cannot be obtained, the host is left alone and cannot be used.

After forcing a match, it begins releasing spores that mutate the host body with Flood DNA.

At this time, the only known subject to be "left alone" by Flood Infection Forms is Sergeant Johnson, due to his being in an incident involving an exploding crate of Plasma Grenades and therefore contracting Boren Syndrome.

In Halo 2, they have learnt how to sacrifice themselves to revive fallen but un-dismembered Combat Forms, greatly increasing their threat level.