The 31-year-old has not raced since Milan-San Remo in March, with his Dimension Data team initially claiming he was suffering from an overuse injury.
However, they have now said he was suffering from “unexplained fatigue” and a UCI blood test revealed he was suffering from Epstein Barr Virus, which caused the fever.
"Mark has been experiencing some unexplained fatigue during training," team doctor, Jarrad Van Zuydam, said in a statement.
"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 two weeks."
Dimension Data had hoped that Cavendish would return to racing later this month at the Tour de Romandie. However that was before Wednesday's full diagnosis was made. It now looks likely that Cavendish will miss both Romandie and next month's Tour of California.
It has also sparked speculation that the 31-year-old could miss the Tour de France, which starts on July 1.
Cavendish has won 30 stages in the Tour de France and won the green points jersey in 2011. He is the second rider on Dimension Data to be diagnosed with Epstein-Barr Virus this year after Jaco Venter in January. The South African recently returned to racing.
With additional reporting from Cycling News
Read the original article on Eurosport: Mark Cavendish diagnosed with glandular fever