Can be. Hirschsprung disease has occasionally gone un-diagnosed for a period of time & some kids have developed a severe enterocolitis & died during the illness. With diagnosis, monitoring& treatment a kid can have a realatively normal life.
Yes. It is usually detected within a few days of birth. It involves disfunction of the large intestine/rectum/anus because the nerves serving those areas don't form properly, so it acts like an obstruction and the babies don't poop. It is a problem that requires surgical intervention. It can be mild, involving only the rectum and anus, or severe, going much farther up.
Sometimes. If not recognized in some infants with long segment disease there can be superimposed enterocolitis and ischemic bowel. Morbidity and mortality increases if propbem not recognized early enough.