According to a detailed account of the soldier's treatment, released by doctors Wednesday, the man was shot five times -- in his buttocks, both arms, back, and knee -- as he ran into South Korea under a hail of bullets from his former comrades.
The UN Command in South Korea said the soldier drove a vehicle near the military demarcation line, the de facto border between the countries, and then "exited the vehicle and continued fleeing south across the line as he was fired upon by other soldiers from North Korea."
More than 40 bullets were fired at the soldier, from pistols and an AK-47, South Korea's military said. South Korean troops did not return fire.
After receiving immediate first aid at the DMZ, the soldier was flown in a helicopter to Ajou University Hospital in Suwon, a town near the South Korean capital Seoul.
He was rushed into surgery "for an emergency operation" 30 minutes after arriving, the hospital said in a statement.
Following the operation, the soldier was admitted to intensive care for about 33 hours, before returning to surgery early Wednesday morning.
"The patient is being administered sedatives in the intensive care unit and mechanical ventilation is maintained using a life support device," doctors said.
More surgery will likely be required on his limbs and buttocks, they added, and he may suffer permanent nerve injury or paralysis as a result of his injuries.
As his small intestine was ruptured by one of the bullets, the risk of infection was high, the statement said, with feces and intestinal parasites found in the man's abdominal cavity and digestive system.
"Contamination by ... feces and parasites was very severe, and the prognosis is likely to be worse than that of general trauma patients because of the long period of time spent in the state of shock due to massive bleeding," doctors said.
"We can not rule out the possibility of an unknown infection due to poor patient status and poor nutrition."
Monday's defection is the third by a member of the North Korean military this year, following two soldiers who fled to South Korea separately in June.
Prior to 2017, there had only been four military defectors from North Korea over the past five years: one in 2016, one in 2015 and two in 2012.
Robert Kelly, associate professor at the Department of Political Science at Pusan National University, said the prominent defection and the shooting was "genuinely surprising."
"It's fairly unusual, I can't think of the last time (a defector was shot at) ... it certainly adds to the tensions," he said.
The defector is currently in South Korean military custody, according to a statement.
"Our military has raised the alert level in anticipation of North Korean provocation. The military is maintaining a full readiness," the country's Joint Chiefs of Staff said.
The North Korean military has more than 1.2 million active soldiers and a further 7.7 million in reserves. It is one of the largest ground forces in the world.