What exactly is hirschsprung's disease?

Lack of nerve cells. Aganglionsis or hd is the result of failure of the bowel to relax. Peristalsis or progression of contractions of the intestine are halted wherever the rectum/colon fails to relax to allow contents to move forward. As a result, child cannot evacuate stool/gas and becomes distended, can become very sick. This can be corrected. In 75%, the bad bowel is in the rectum or just above.
Constipation. Hirschsprung's disease is a congenital disease. You are born with it. It causes constipation and /or obstruction due to non functioning of intestinal nerves. It is treatable, but may require surgery to diagnose and treat.
Lack ganglion cells. Hirschsprung’s disease (congenital megacolon) is caused by failed migration of colonic ganglion cells in utero. Varying lengths of the distal colon are unable to relax, causing functional colonic obstruction.Can vary from very short segment to entire large bowel. Obstruction noted usually in infancy, Sometimes recognition is later due to intractable constipation.