See photos of fingernail conditions that warrant medical attention.
Spoon nails (koilonychia) are soft nails that look scooped out. Often, spoon nails are a sign of iron deficiency anemia or a liver condition known as hemochromatosis, in which your body absorbs too much iron from the food you eat. Spoon nails can also be associated with heart.
Children who suck their thumb or bite their nails 'less likely to develop adult allergies'. The study goes even further to say for those who have.