Nail fungus infections, otherwise known as Onychomhycosis are unsightly and an embarrassment. On the toenails, it is not as bad as on the fingernails since it is easier to hide. What are the causes of fingernail fungus infections is a subject of debate. However, there is agreement in some areas, as the experts will tell you.Immune system deficiencies could be a cause although it is not known exactly why. There is the question of heredity where immune deficiencies are passed down it the family. P