Alopecia is a skin disease that causes hair loss on the head and elsewhere. It usually starts in childhood, with one or more small, smooth patches.

The body's white blood cells attack hair follicles by mistake, which stops the growth. Most often this happens on the scalp, but anywhere that grows hair can be affected. No matter how much hair is lost, the follicles are still alive.

Hair can and does start regrowing without treatment. Doctors also have medicines that can help wake up the follicles.