A North Korean soldier who was shot fleeing across the border had an extremely high level of parasites in his intestines, his doctors say.
The defector crossed the demilitarised zone on Monday, but was shot several times by North Korean border guards.
Doctors say the patient is stable - but "an enormous number" of worms in his body are contaminating his wounds and making his situation worse.
His condition is thought to give a rare insight into life in North Korea.
"I've never seen anything like this in my 20 years as a physician", South Korean doctor Lee Cook-Jong told journalists, explaining the longest worm removed from the patient's intestines was 27cm (11in) long.
How do you get a parasite?
Humans can get parasites through eating contaminated food, by being bitten by an insect or by the parasite entering through the skin.
In the case of the North Korean defector, the first case is most likely. Parasites entering the body via contaminated food are often worms.
The North still uses human faeces as fertilisers.If these faeces are untreated and fertilise vegetables that are later eaten uncooked, the parasites get into the mouth and then the intestines of the person.
While some don't cause any severe symptoms, others can be life threatening, explains Prof Peter Preiser from the School of Biological Sciences at Nanyang Technological University in Singapore.
"What they all do is take nutrients away from your body," he told the BBC. "So [even] if most of them might go unnoticed, they all indicate a poor health status. To put it simply: people who have parasites are not healthy."
What does it tell us about N Korea's health care?
"North Korea is a very poor country and like any other poor country it has serious health problems," Prof Andrei Lankov of Kookmin University in Seoul told the BBC.
But Mr Lankov points out that compared to other countries with a similar per capita income - like Bangladesh for instance or many African countries - the population in North Korea is a lot healthier than one would expect. The country's life expectancy is well above average considering the level of economic poverty, he says.
"North Korea does not have the resources to have a modern medical system," he says. "Its doctors are relatively poorly trained and have to work with primitive equipment."
Yet given that there are relatively many doctors, the country does manage to perform rather well in its dire economic situation, he argues.
Are the parasites endangering the soldier?
Yes. Parasites are not a great thing to begin with but given he is already in a critical condition, they are making a bad situation much worse.
The soldier was shot at least six times when North Korean border guards opened fire on him, as he ran across the Joint Security Area in Panmunjom, to the South Korean side.
The young soldier was rushed by helicopter to a hospital in South Korea and has undergone emergency surgery.
Doctors say his wounds are serious - and there is a severe contamination of the wounds by both parasites and faeces.
"We are paying close attention to prevent possible complications," Surgeon Lee Cook-jong told reporters, explaining that "an enormous number of parasites" including roundworms had been found in the small intestine.
If the intestines are disrupted and parasites are released into the body cavity, they can be a lot more dangerous and much more complicated to treat.
Also, he is likely to have had the parasites for a long time. This means "that patient will generally be in a weaker state so any wounds and surgery will become more dangerous," explains Mr Preiser.
Is this a problem particular to North Korea?
Parasites, especially worms, are thought to be widespread in North Korea.
However, they also affect many developing countries where diets include uncooked vegetables fertilised with faeces.
While there are ways to treat faeces so it can be used as a safe fertiliser, many poor countries neglect to do so.
In the current situation in North Korea, "this seems to be a sort of neglect here - and one that's been discussed for years," says Mr Lankov.
The poor health and nutrition has widespread consequences "but North Korea does not admit this because they fear this will affect their image too much."
Should there ever be a reunification of the two Koreas though, there would be massive health problems for officials to deal with for decades to come, Mr Lankov warns.