The Infection Form is quite small, and vulnerable to physical attack. To compensate, it is very numerous, and rather fast. It can move across the ground, or along walls, and for short distances is capable of large jumps. If it comes in contact with an energy shield, it explodes, or "pops"... but when there is no shield it will latch on to the host and embed itself in the host's chest cavity, taking over all motor functions.
-- from halo.bungie.org
's "Anatomy of a Flood" section.
Specific Link: Infection Form