In early 2019, Erasmus was diagnosed with microscopic polyangiitis with granulomatosis and underwent chemotherapy before and during the World Cup.
Erasmus' doctor Johan Theron told Afrikaans publication Rapport that the side effects made the coach weaker, but was essential as the disease was potentially fatal.
"He really doubted at one stage whether he would be able to push through with his position as Springbok coach," Theron said.
His chemotherapy doses were lowered so that he could give his full attention to the Springboks during the World Cup.
Dr Theron added that Erasmus decided to keep his illness and treatment out of the public domain as he didn’t want the news to serve as a distraction.
The Springboks beat England 32-12 in the final to win their third World Cup title in what was Erasmus' last game as coach.
He stood down and moved into his role as director of rugby on a full-time basis, handing the coaching reins to Jacques Nienaber.
South Africa Rugby confirmed that Erasmus finished treatment in March this year and that his family are following strict precautionary measures to avoid being exposed to COVID-19.
"He thanks people for their concern but wishes to keep the matter private," SA Rugby added.