Syndrome K: The disease you've never heard of that actually saved lives during WWII.

In the second half of 1943, the Nazis invaded Italy.

World War II had been declared in 1939, and since 1940 members of the Schutzstaffel (SS) had been posted to camps all across Europe. What began as labour camps, quickly became concentration camps, where millions of European Jews, homosexuals, political dissidents, Soviet prisoners of war, gypsies, people with disabilities, Jehovah’s Witnesses and various other minorities were brutally and senselessly murdered.

Although there was no way to know the extent of the persecution, European Jews understood that they were the primary enemy of the Third Reich. And once Nazis crossed the Italian border, a number of Italian doctors, among them Vittorio Sacerdoti, and surgeon Giovanni Borromeo, developed a genius plan.

As Nazis raided a Jewish ghetto just outside Rome, doctors hid a number of Italian-Jews inside Fatebenefratelli Hospital.

These patients were admitted to hospital with Syndrome K, a disease believed to be highly contagious, disfiguring and potentially lethal. Doctors likened the condition to cancer or tuberculosis. If one person entered a deportation train with Syndrome K, they had the potential to infect everyone on board - including Nazi soldiers.

What Nazis didn't know - is that there is no such thing as Syndrome K.

The name was coined by a anti-Fascist physician named Dr. Adriano Ossicini, who needed a way of filing patients who were, in fact, Jews in hiding.

“Syndrome K was put on patient papers to indicate that the sick person wasn’t sick at all, but Jewish,” Ossicini told Italian newspaper La Stampa in 2016.

“We created those papers for Jewish people as if they were ordinary patients, and in the moment when we had to say what disease they suffered? It was Syndrome K, meaning ‘I am admitting a Jew,’ as if he or she were ill, but they were all healthy... The idea to call it Syndrome K, like Kesselring or Kappler, was mine.”

In 2004, Dr Sacerdoti told the BBC, once the Nazis heard a patient was inflicted with Syndrome K, "they fled like rabbits".

Sacerdoti's 10-year-old cousin was among those saved by the fictitious disease.

The fear of Syndrome K was so great, that Nazi troops didn't inspect the hospital rooms of sufferers. Jewish children were coached by doctors to cough loudly and turbulently, in case a soldier spotted them.

Estimates for how many lives were saved range from 20, well into the hundreds. During the two year Nazi occupation of Italy, at least 10,000 Italian Jews were deported, and most never came home.

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For 60 years, Syndrome K was lost to history. It has only been since doctors from Rome's Tiber River have spoken about their comprehensive plan, that historians have learned that 'Syndrome K' written in the case files of patients, was in fact, a lie.

It might just be the only disease to have ever actually saved lives.