Stoke defender Robert Huth has suspected meningitis, the club have announced.
The 27-year-old German was sent to hospital after reporting for training suffering from a mystery virus on Monday.
The Potters posted on their official Twitter account at around 2330 last night: "The club can confirm that Robert Huth has suspected meningitis and that they are awaiting the outcome of further tests."
Speaking to club website http://www.stokecityfc.com after Monday night's 1-1 pre-season friendly draw at Torquay, manager Tony Pulis said: "It's a major concern so close to the start of the new season because Huthy is such an important figure for us.
"It takes a lot to knock Huthy back, but he was clearly not well, so we have sent him to hospital and will hopefully have the results of the tests on him tomorrow.
"We're just hoping he will be okay and it's not too serious."
Stoke begin their Barclays Premier League campaign at newly-promoted Reading on Saturday, August 18.
Huth arrived in English football as a teenager with Chelsea in 2001, winning two Premier League titles with the Blues before moving to Middlesbrough in 2006.
After transferring to Stoke in summer 2009 in a deal initially worth £5million, he was named the club's player of the year and helped the Potters reach the FA Cup final in the 2010/2011 season.
Huth has played 19 times for his country and was part of the Germany squad which finished third at the 2006 World Cup on home soil.