What causes a hernia?

Nature/Nurture: Most hernias develop due to a "birth defect" of sorts, where natural holes in the abdominal wall present in fetal life fail to close. This may manifest itself at birth or require years of "exertion" before the hole reaches a size large enough to be evident. Other hernias are strictly "wear-and-tear" phenomena, although it is believed that they occur in-part due to inherently weak tissue.

See below: It is a defect or weakening in the abdominal wall (usually near the groin or in the lower abdomen). This can be caused by many things including past surgery, injury, excess weight, and others. Then a piece of intestine can penetrate the defect. The risk is strangulation and death of the intestine. This can happen anytime - that's why it should be treated.