Nails become yellow and hollow, a condition that can have a number of causes. You can consider gray nails, which is a fungal infection and contagious, where the fungus eats away at the nail and makes the nail hollow. The performance often starts from the sides or ends of the nail plate, most of the first mild inguinitis, due to the lack of timely treatment will gradually evolve into a chronic gray nail. Inflammation of the nail grooves can be seen as pits or grooves on the surface of the nail, which may remain unchanged or gradually involve the nail root. If the nail is infected, not only will cracks form, but the nail will also become brittle or thickened to a brown or black color, thus making the nail hollow. This condition can be treated with oral topical medications or surgical nail removal. Toenails grow slowly and it takes about 300 days to recover to completely replace a new nail.