When the virus started spreading, the first ones to get hit were those with a weak immune system. Namely, newborns, babies and little children. However, as these groups are still growing, the virus, instead of simply taking control over the targets nervous system, it becomes an integral part of the child's DNA. These infections are particularly diabolical, as the children show no symptomes, lulling their parents into thinking that "My kid is safe!". (Especially common with newborns, under a year old.) As they start to grow up, they become increasingly "weird", they start talking to themselves, twitching, and complaining about "the voices". This is the virus talking, lodged deep within the sub-conscious. The parents will now be forced to kill their child, as it is clear that it has been infected, and pose a great risk to any survivors in the group.
But, sometimes, the family is isolated from contact with survivors, be it because they escaped to some remote location, they are incredibly paranoid etc. That is when a Nightmare is created. When the child hits puberty, the virus really kicks into overdrive, and augments the bodys own hormone production, to increase muscle mass, faster reflexes, greater vision and hearing, as well as taking over the entire brain of the child, basically creating a zombie super soldier, with full cognitive thinking and ability to learn. (Depending of course, how well his parents raised him)