Diseases of the Foot
This is a condition in which a spur or splinter of nail, or perhaps a serrated section of the edge of the nail has pierced the skin and penetrated the tissue. When you touch that spot it will feel like a needle is sticking you. The offending piece of nail penetrates further and sets up an acute inflammation in the surrounding soft tissue, which almost invariably becomes infected.
Clean the area with a mild antiseptic, cover the area, and get the person to medical aid soonest. Especially if that person is diabetic it is ever more urgent.