The Dane underwent blood tests after his abandon on the first stage of the Settimana Lombarda, at the start of this month. He told Danish website Politiken.dk that the virus may account for a string of mid-field results that have marked his 2010 season.
"The tests proved that I have mononucleosis, and that I probably have had the disease in the body for two months," he said. "It does provide a thorough explanation of why I never have been close to hitting top form, but simply felt ill and completely lethargic without really understanding why."
Rasmussen's best result this season was an eighth placed finish overall at the Tour de San Luis in Argentina. He was unable to carry that momentum across to Europe, where he has recorded DNFs in is last two races - Settimana Coppi e Bartali and Settimana Lombarda.
Although relieved to have pinpointed the cause of his lacklustre results sheet, he faces an uncertain period on the sidelines.
"On the one hand, it is of course deeply frustrating to be sidelined at a time when I face some of the most important races of the spring," he said.
"On the other, it is nevertheless reassuring to have become quite clear on what is wrong. Now there's nothing else to do but wait to race again until I recover, and when that happens, it is impossible to say at the moment. "
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Rasmussen, 35, will continue to train in preparation for the all-clear from his doctors. "I continue to train, but can't push myself to the limit," he said.
"I feel it is important that I keep myself going, so I'm not handicapped any more than necessary when I get the green light to start racing seriously again."
