Mononucleosis, very unpleasant blood decease. It also affected the career of Ancic.
Here's a quote:
"In March 2008, Federer revealed that he had recently been diagnosed with mononucleosis and that he may have suffered from it as early as December 2007. Federer also had an illness related to food poisoning prior to the start of the Australian Open. He noted, however, that he was now "medically cleared to compete"
In that aspect his performance last year was pretty decent, but I'm more concerned of this year's situation about him.