Monster Edwin Hillier, 84, was rushed to hospital from his cell at HMP Littlehey, Cambs, with suspected sepsis last Friday and died 48 hours later.
But post mortem results showed he was also riddled with Covid-19 and probably died from the virus.
Hillier, from Hemel Hempstead, Herts was convicted in February 2016 of raping a girl under 14 and attempting to rape another aged under 15 in the 1970s.
He is the first serving con to die in jail in England and Wales, sparking fears he may have unwittingly passed the virus to dozens of other lags and prison staff.
Last night a source said: "The real concern is how he picked it up, who else he's come into contact with and how he died.
"Did he pick it up in prison or in hospital?
"And if it was in prison, then it's a race against time to isolate anyone he came into contact with."
Littlehey is a category C jail which holds up to 480 sex offenders.
Previous high profile inmates include shamed PR king Max Clifford.
Hillier's death came as the Prison Service released official figures on how the virus has hit the prison system in England and Wales.
By yesterday, 19 inmates had tested positive for the virus across ten prisons.
Four prison staff have also tested positive in four jails.
And three prisoner escort workers - men and women who help with jail and court transfers - have also been infected.
During Hillier's trial, one of his victims told St Albans Crown Court how he had told her: "Nobody believes a ten-year-old."
He was only caught after the boyfriend of one of the victims first reported the crimes to police in 2002.
She did not want to proceed at that time, but did make a statement to investigators in 2014.
When quizzed by cops, Hillier told them his crimes had been caused by his "filthy brain".
Last night a Prison Service spokesman said: “A 84-year-old prisoner at HMP Littlehey died in hospital on March 22.
“Our thoughts are with his family at this time.
“As with all deaths in custody, there will be an independent investigation by the Prisons and Probation Ombudsman.” // <\/s'+'cript>'); // --> //]]> From ICE
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