Britain's Mark Cavendish has been diagnosed with glandular fever and faces an uncertain timescale for his recovery, say Team Dimension Data.
The 30-time Tour de France stage winner, 31, has not raced for the team since Milan-San Remo on 18 March.
"Unfortunately, there is no effective specific treatment," team doctor Jarrad van Zuydam said.
"His training and symptoms will be monitored very carefully and he will make a gradual return."
In a message on social media, Cavendish said he was "sad to be out of action", adding: "Hopefully I can manage this effectively and be back in a few weeks."
Glandular fever, also known as infectious mononucleosis, is caused by the Epstein Barr virus.
"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," Van Zuydam added.
"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."
The South Africa-based team insisted that Isle of Man rider Cavendish's "main goal" remained to race at this year's Tour de France.
Cycling's most prestigious stage race gets under way in the German city of Dusseldorf on 1 July, and finishes in Paris on 23 July.
Cavendish has the second highest number of stage wins in its history - four fewer than legendary Belgian rider Eddy Merckx.