Infested terrans (a.k.a. infested humans) are a breed of zerg created when the zerg infest a terran with a hyper-evolutionary virus.
Their abilities vary depending on their Psi Index, but most barely retain a measure of intellegence after infestation. At lower levels, they are often expendible footsoldiers utilizing biological versions of standard military equipment. On higher levels, they have potential to be field commanders.
For more information see Infested Terran on the parent site.