The immortals are human in all but how they age, meaning: They are born normally. They grow and age normally until they reach their peak, like their mid twenties to thirties or what have you. Once they reach their peak they stop aging, their cell's are replaced as rapidly as they die, this does NOT include huge losses of cells such as limb loss, so if the immortal loses an arm or a leg, it's gone for good. They scar normally and they also can still get sick, though I would imagine that they would heal at a more rapid rate.