The 31-year-old has not raced since the Milan-San Remo one-day event on March 18.
The 31-year-old has not raced since the Milan-San Remo one-day event on March 18 and his absence was at first attributed to a right ankle injury.
But Team Dimension Data said in a statement: "Recent blood work has revealed that Mark Cavendish suffers from infectious mononucleosis caused by the Epstein-Barr virus."
Infectious mononucleosis is also known as glandular fever.
"Mark has been experiencing some unexplained fatigue during training," added team doctor Jarrad van Zuydam.
"Unfortunately, there is no effective specific treatment against the virus but rest will be required to aid his recovery.
"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."
A 30-times Tour de France stage winner and one of the outstanding sprinters of his generation, Cavendish won his first Olympic medal when, in his third Games, he took silver in the omnium at Rio last year as he combined his road career with a track programme.
Cavendish is just four wins away from equalling Tour de France great Eddy Merckx's record of 34 stage wins.
This year's tour starts in the German city of Dusseldorf on July 1 and Van Zuydam said: "His training load and symptoms will be monitored very carefully and he will make a gradual, step-wise return to full training and racing."