Jimmies (also known as Zombies)
Because Carol is not the most informed of narrators, she cannot refer to these "monsters" by anything other than the pop culture term of Zombie, which isn't far from the truth. These people have taken on a greenish tint, uncontrollable anger, and an insatiable hunger for meat of any kind. They cannot be stopped by anything but a severely damaging blow to the brain or spinal cord.
Their condition is spread via saliva or blood contamination, and the process for a normal human to "turn" may take anywhere from several hours to several days.