Untreated cholera causes profuse, watery diarrhea leading to rapid loss of fluids and electrolytes. This significant fluid loss results in severe dehydration, which reduces circulating blood volume causing hypovolemic shock. If not promptly managed with rehydration, this shock can progress to multi-organ failure and death. The primary life-threatening complication is therefore dehydration shock, making timely fluid replacement crucial. Other complications like renal failure or cardiac arrest may occur secondary to severe shock but are not the initial major issue.

Reference: Harrison's Principles of Internal Medicine, J. Larry Jameson, 20th Edition.