The 47-year-old who transformed the national team from a laughing stock to world champions within 18 months struggled with a rare autoimmune disease last year.
Called microscopic polyangiitis with granulomatosis, it can affect organs such as the kidneys and lungs and also sinuses and trachea.
While the disease is not cancerous, the coach suffered a rare strain of it that was potentially fatal, according to his doctor, Johan Theron.
“Rassie was treated for a serious condition in 2019, from which he has recovered,” a South Africa Rugby spokesperson confirmed.
“He thanks people for their concern but wishes to keep the matter private.”
Afrikaans-language newspaper Rapport reported that Erasmus required chemotherapy before the World Cup kicked off last September and during the 44-day campaign.
The coach kept his condition secret from most throughout an amazing turnaround of the national team that climaxed in a 32-12 triumph over England in the World Cup final.
Recently retired Springboks loose forward Francois Louw said Erasmus showed no signs of a serious illness while coaching the country to a record-equalling third World Cup title.
According to the Netwerk24 website, the senior campaigner was among a handful of players who knew the Springbok coach was sick.
Louw, who came off the bench in the final to replace skipper Siya Kolisi, said it was never obvious to the squad that something was wrong with Erasmus.
“Rassie is the type of person who does not want everybody talking about him because his outlook is that everything is about team success and not individual issues.
“He is the type of person that does not look for sympathy or empathy. Only his inner circle were aware of his condition.
“When I recall his work as head coach before and during the World Cup, you could never tell that anything was wrong with him.
“I do not think most of the squad were aware that he had a serious illness. I was one of the few who knew.
“Rassie’s way of handling it was to not let the squad get concerned about it. He is a tough guy. There is a reason why he played Test rugby.”
Erasmus was capped 36 times by the Springboks as a loose forward in a five-season Test career from 1997 and scored seven tries.
Before taking charge of South Africa, he coached provincial teams the Free State Cheetahs and the Western Province/Stormers in his homeland and Irish outfit Munster.
“Technically, there is not a better coach in South Africa than Rassie,” according to Springbok legend Victor Matfield. “In fact, he is one of the best in the world.”