Mark Cavendish has been diagnosed with the Epstain Barr Virus, which causes infectious mononucleosis - more commonly known as glandular fever.
As a result the Dimension Data rider will have to take an enforced break before making a gradual return to training and racing.
As well as the Epstein Barr Virus, which was discovered in recent blood tests, Cavendish is also suffering from an ankle injury which he picked up at Tirreno-Adriatico. Although he rode Milan-San Remo (finishing 101st), the injury, which his team describe as being from 'overuse', kept the sprinter out of both Scheldeprijs and Paris-Roubaix last week.
With the Epstein Barr Virus now added to the pre-existing ankle injury, Cavendish will now need a longer period of rest than had originally been anticipated.
'Mark has been experiencing some unexplained fatigue during training,' said Dimension Data team doctor Jarrad Van Zuydam. 'Recent blood analysis has revealed him to have infectious mononucleosis caused by the Epstein Barr Virus. Unfortunately, there is no effective specific treatment against the virus but rest will be required to aid his recovery.
'His training load and symptoms will be monitored very carefully and he will make a gradual, step-wise return to full training and racing.
'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 2 weeks.'
Dimension Data say that the main goal of the year for Cavendis remains the Tour de France, which begins on July 1st in Dusseldorf.
After notching up four stage victories in the 2016 edition of the Tour, the manxman brought his tally up to 30 career Tour de France stage wins - 4 behind Eddy Merckx's record.