(CNN)British cyclist Mark Cavendish, one of the sport's leading sprinters, is to take a break from the sport "due to the presence of Epstein-Barr virus," says his team.
Cavendish, who is the 2011 world road race champion, is second on the Tour de France's all-time stage win list.
He has won 30 Tour stages, four behind the record of Belgian Eddy Merckx, but the Isle of Man rider failed to add to his tally in 2018 and exited the race after missing the time limit on stage 11.
"The medical results have indicated, too, that Cavendish will also have been unknowingly training and racing with EBV over recent months, and as a result of these findings he has been advised to rest in order to fully recover ahead of a return to training," said a statement from his Team Dimension Data outfit.
In 2017, Cavendish was diagnosed as suffering from mononucleosis, more commonly known as glandular fever, a viral illness that can linger for weeks, months or even years.
It is caused by EBV, which is a common virus that is spread through bodily fluids, such as saliva -- hence its nickname the "kissing disease."
No timescale has been given for the 33-year-old's return to racing.
"This season I've not felt physically myself and, despite showing good numbers on the bike, I have felt that there's been something not right," said Cavendish.
"Given this, and on the back of these medical results, I'm glad to now finally have some clarity as to why I haven't been able to perform at my optimum level during this time.
"Having received expert medical advice as a result of the findings I've been advised to take a period of total rest in order to fully recover.
"I'm now looking forward to taking the time necessary in order to get back to 100% fitness before then returning to racing again at peak physical condition."