It could be a deformed baby, a premature baby, or a Siamese twin baby, doesn't matter. It would still be a human. Let's move to the other end of the spectrum, old age. Does it matter that an extremely elderly person losses all control of his faculties, no. This being is still a human. Such a being may not have the ability to enjoy the faculties, which one time brought so much joy to life, but that's a question of quality rather than humanness itself. All ethical considerations aside, I really can only think of death as being a situation in which humanness ceases to be of a particular being. One might object and say that after a human dies, he's still human; it's just that he's a dead human. But consider this: if a man/woman were to die, one could say that he/she was a good man/woman, thus implying that he/she is no longer a man/woman. Now, man and woman exhaust the list of things human when speaking of the categories, so if something is not a man or woman anymore, then it is reasonable to conclude that the being in question is no longer human. On the grounds of this one could assume that the universal humanness always accompanies human babies at least from birth onward until death (I would try to take humanness earlier in the development of a baby, but that would be another lengthy discussion).
I still feel that there is some clarification to be had in respect to the latter part of the above statement, so let's take it further. Let's assume that all human beings died. Nowhere in the universe would there be such a thing as a human, nor would there be any trace of humans left. Does this mean that humanness itself would vanish as well? To answer this, we must look into the nature of universals. If universals were independent of objects, if they were something else altogether and on another plane of being, then they would be considered transcendent. That being the case, the answer to the question above would be no.
