This is true.
High school story. I was on the cross country team with a couple of really talented twins. They were on varsity in 8th grade. Really good runners. Anyways, they were both pretty much equally fast. Unfortunately, one of them got a real bad case of mono as a freshman and he was actually never the same again. He was about 2 minutes slower in a CC race to his brother, who went on to win 3 state titles in CC and track and went on to run collegiately at a D1 level. The one that suffered mono was still a good runner, just not the same as he once was.
Long story short ... depending on the severity of mono, it can be real tough to come back from.
I hope the cases with Forbs and Mac aren't severe and they'll be back soon ...