James Stewart has withdrawn from the upcoming Genoa and Brisbane Supercrosses due to medical reasons, ending his "world tour" prematurely and putting a question mark on his health with the AMA Supercross season just eight weeks away.Stewart, who pulled out of the final night of the Supercross of Paris in Bercy with suspected food poisoning, returned home after Paris and a medical evaluation by his doctors have found his "immune system to be dangerously compromised." Stewart has been advised by his doctor and team to discontinue any strenuous activity and not travel out of the country at this time, according to a release issued by his publicist today."I am really bummed about not being able to race in Italy and Australia," Stewart said in the release. "I've had such a great experience racing internationally. The fans are incredible and the race organizations really go out of their way to support me racing. I really wish I could still go but after talking with my doctors and team it's not in my best interest to jeopardize my health before the long Supercross season starts."His Team San Manuel team manager concurred: "At this time, James will be focusing on getting healthy and preparing to defend his Supercross championship title. Taking time to improve his health and well being is our first priority."According to the release, Stewart will continue to undergo medical evaluation and refrain from any kind of strenuous training activity until his doctor and medical staff advise him that he is fit to resume.