Combat forms are relatively intact sentient hosts under the control of an infection form that has nested within. Though sections of the host's body are consumed and converted into Flood cells, some limbs and sensory organs are retained to utilize available weapons, tools, vehicles, equipment, and even starships. The infection form does augment its new body and repair minor damage in order to better serve as a living weapon, but ultimately the combat form is utterly disposable.
The infection form driving the host body does not feel pain or fear, though the same cannot be said for the host. Worse, in some horrific circumstances the infection form is unable to completely dominate the original personality and the victim remains fully aware of their irrevocable transformation and use as a tool of the parasite.