Within the Carrier Form the spores (Infection Form) are created/nurtured. The Carrier form come in two flavors - they're either Flood-infected Grunts (the Flood don't consider Grunts an effective combat form), or they're Warrior forms (either human or Covenant) so badly damaged they're no longer fit for battle. The reason their legs are so short is because they're missing the bottom half, in most cases - their 'feet' were once knees. In either case, their primary purpose is the same: they serve as a portable incubator for future spores. The Carrier Form is quite slow, but responds to damage by simply releasing its progeny in an explosive rush; it is capable of killing or disabling nearby beings (friend or enemy, it's indiscriminate) when it detonates. It will detonate itself if it gets close enough to an enemy target.
-- from halo.bungie.org
's "Anatomy of a Flood" section.
Specific Link: Carrier Form