Final answer:
A nail lost due to disease is likely to grow back if the nail matrix is healthy, as the nail matrix contains stem cells needed for nail growth. The nail is mainly comprised of a protein called keratin.
Step-by-step explanation:
A nail that has been lost because of disease is likely to grow back if the nail matrix is intact and healthy. This is because a nail grows from a deep layer of living epidermal tissues, known as the nail matrix, located at the proximal end of the nail. The nail matrix contains stem cells that continuously divide to allow for nail growth. The growing nail begins as the nail root and then forms the nail plate as it emerges from the epidermis. Therefore, if a nail has been lost due to disease but the nail matrix remains unaffected, the nail can regrow.
To link to the other provided information, nails, like hair, grow continuously, and the nail bed is not the middle portion of the nail plate (this is false) but the layer of skin upon which the nail plate rests. Nails are primarily composed of keratin, a protein that also makes up the structure of the hair and outer layer of skin.