England are assessing the fitness of opener Alex Hales who is suffering from a virus, the day before the third Test with South Africa begins in Johannesburg.
Hales was able to have a net session on Wednesday, before he was whisked away by the medical staff.
England captain Alastair Cook said: "He has had a hit and will go straight back so he doesn't infect the rest of us. So we'll have to wait and see on him. But he has had a hit so he can't be too bad.
"I can't do anything about it as a captain. The medical team are on it. It seems to strike a bit randomly. Nick had it the other day."
Middlesex batsman Nick Compton has fully recovered from his stomach bug and will be able to take his place at No 3 in the batting line-up.
If Hales is unfit, Compton could open, with Gary Ballance coming into the middle order.
Cook has cautioned England not to get ahead of themselves as they seek to wrap up the series.
The tourists lead 1-0 with two Tests remaining, having comprehensively won the opener in Durban before an entertaining draw in Cape Town.
And Cook said: "We're in a really good position and it's an exciting time to play, on probably two result wickets for these two games.
"We need to win one of them to win the series. It's a great position to be in, we've played some good cricket so far on tour, but the danger of saying you only need to win one is you start looking too far ahead.
"We need to do a lot of good things to put pressure on South Africa and win the game. The results will look after themselves if we play well."
A seamer-friendly wicket is expected at the Wanderers, and the home side will field an all-pace attack having released off-spinner Dane Piedt.
Hardus Viljoen is in the frame, but Ben Stokes' all-round ability means England will have four seamers plus Moeen Ali in any case, and Cook said: "I am very comfortable with this team playing in any conditions.
"If the ball does fly through and seams around, we have got a pretty good seam attack as well and if it does get warm and does spin a little bit, we have got that option."
Live coverage of the third Test between South Africa and England begins at 8am on Thursday on Sky Sports 2 HD.