This is because of the illness, however, the rest of the squad is able to participate.
Meanwhile, Joe Denly, who was struck down by the bug, has now recovered from the same.
England make wise decision to call Overton and Bess
The Boxing Day Test between the two sides begin at Centurion.
England have taken the precaution and made a wise call of calling Overton and Bess.
Both players have Test experience. Bess played two matches last summer against Pakistan while Overton was selected for the Old Trafford Test during the Ashes 2019.
Bess could play at Centurion should Leach not recover.
ECB thanks CSA for being flexible
A spokesperson for the England and Wales Cricket Board said: "ECB would like to take this opportunity to thank CSA for their flexibility on this situation and to emphasise that under normal circumstances we would have wanted to play the match first-class, but illness in the camp has made this very problematic."
Test Championship: Crucial series for SA and England
England, who collected 56 points from the Ashes 2019 series, face a crucial away test next.
They play against South Africa in a four-match series, starting December 26.
It is a pivotal series for Joe Root's side and the points are there for the taking.
For the South Africans, they were hammered 3-0 by India in October and haven't opened their account yet.
England's practice game gets downgraded
Most notably, the bug's effect on the England squad has seen the practice game at Willowmoore Park get downgraded from a First-Class match. Instead, both teams can rotate their line-ups, however only 11 players can bat or field at any one time.