NASA and the European Space Agency (ESA) released the news that ESA astronaut Hans Schlegel, part of the space shuttle crew Atlantis that launched Feb 7 on mission STS-122, was pulled from a space walk due to illness. Both NASA and ESA emphasized that Schlegel's illness was not "life-threatening" or "contagious". Due to privacy concerns, NASA usually won't release any details on an astronaut's heath, either when they are on earth or in orbit, so I don't expect to see anything specific come out about Schlegel's problem. However, with the illness occurring so early in the mission, I wouldn't be surprised if it's just a bad case of motion sickness. (I think you can safely classify a case of the pukes as "non-contagious and non-life threatening", though I'm sure there are times you wish you were dead it gets so bad.) It's pretty widely known that some astronauts don't acclimate to zero-g as well as others, and it may take a couple days for their stomachs to settle down and their head to stop spinning. After that initial period of instability, their body adjusts and they can pretty much function normally.  Since the first spacewalk was scheduled early in the mission, NASA and ESA probably didn't want to take a chance of sending a sick astronaut outside.

Schlegel's Bio indicates he did fly in space before, so this isn't his first time in zero-g. However, that flight was almost 15 years ago, and like anything else you haven't done for a long time, your body has to learn all over again to re-adjust. The primary purpose of STS-122 is to deliver the ESA Columbus Laboratory Module to the space station.