The players have been briefly affected by a 'Norovirus'. The virus is not food-related, but causes gastric pain over a 24 to 48-hour period.
England doctor Simon Kemp said: "The symptoms have affected a small number of the England squad to date.
"One of the positive features about this sort of viral infection is that it typically develops and settles relatively quickly."
Under-strength England face a tough enough challenge without possible disruption to their team ahead of tackling South Africa in Bloemfontein.
They have not won away from home for 15 months, while the Springboks are savouring a feelgood factor generated by two South African teams - the Sharks and Bulls - contesting last Saturday's Super 14 final.
Not that England head coach Brian Ashton's team of "part-timers, plumbers and decorators," - as one South African newspaper labelled them - intend making life easy for their hosts, who are overwhelming favourites to win the series 2-0.
England number eight Nick Easter said: "You like a big challenge in the forwards.
"Hopefully, we can match traditional South African forward power with some English forward power, and I am really looking forward to it.
"We've got the jersey at the moment, and possession is 9/10ths of the law, as they say.
"We've got a massive chance to prove what we are about in a southern hemisphere country and in the heartland of South African rugby, which is where the Tests will be played (Bloemfontein and Pretoria)."
And the defiant mood is mirrored by 35-year-old Bristol hooker Mark Regan, who returns to England duty three years after walking away in protest at being overlooked by Sir Clive Woodward's successor Andy Robinson.
Regan is realistic enough to know he would probably have missed England's trip had Ashton not left behind players from European finalists Leicester, Wasps and Bath.
But he said: "The reason you play the game is because it's all about challenges.
"Personally, you can't get any bigger than playing South Africa - the most physical team in world rugby - in their own back yard."