Mark Cavendish has been diagnosed with infectious mononucleosis - better known as glandular fever - caused by the Epstein-Barr virus, with his Dimension Data team unwilling to put a date on his return.
The Manx sprinter had already been suffering from an ankle injury that ruled him out of Paris-Roubaix, but will now require a longer period of rest than initially anticipated.
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"Mark has been experiencing some unexplained fatigue during training. Recent blood analysis has revealed him to have infectious mononucleosis caused by the Epstein-Barr virus," team doctor Jarrad Van Zuydam said in a statement.
"Unfortunately, there is no effective specific treatment against the virus but rest will be required to aid his recovery."
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The team says that Cavendish's main goal remains the Tour de France, but were not able to give a date for his return, and he is now expected to miss both the Tour de Romandie and the Tour of California.
"His training load and symptoms will be monitored very carefully and he will make a gradual, step-wise return to full training and racing," Van Zuydam continued.
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"It is difficult to give an accurate estimate of when we can expect him back at full fitness but we are hopeful of a significant improvement of his symptoms over the next two weeks."
Cavendish is far from the first professional cyclist to suffer from the illness, with Team Sky's Beñat Intxausti having contracted the virus in February 2016, and having only competed in two races since.
