"Recent blood work has revealed that Mark Cavendish suffers from infectious mononucleosis caused by the Epstein Barr Virus," team doctor Jarrad Van Zuydam said in a team statement.
The 31-year-old has not raced since Milan-San Remo on March 18 after picking up what the team described as "an overuse injury", and underwent tests after suffering unexplained fatigue during training.
Blood tests revealed the virus, for which the only cure is rest, the statement said.
Cavendish is likely to miss the Tour de Romandie later this month but recovery is expected within two weeks and the 30-stage Tour de France winner hopes to be ready for the start of this year's event in Duesseldorf on July 1.
"[He] will need a longer period of rest than originally anticipated, as he is currently recovering from an overuse ankle injury simultaneously. The main goal for Cavendish remains the Tour de France this year," Van Zuydam said.
Cavendish experienced an arduous 2016 in which he competed 71 days on the road, targeted an Olympic gold medal in Rio - he ended up winning a silver in the omnium - and competed in several track events.
(Reporting by Neil Robinson; editing by Mark Heinrich)
