"I'd rather leave an explanation of Art's situation up to Art," Harbaugh said, according to The Baltimore Sun. "It's not anything that's like life-threatening or anything like that. It's non-football related. We've got to get some tests done to make sure that he's OK. Until the tests come out the right way, he's not going to be able to play.
"It's something I've been told now that they can't tell you how long it's going to be. It could be a week, it could be indefinitely. We don't think it's going to be the second part. He's going to see a specialist here on Wednesday and he can explain to you exactly what it's about, to whatever extent he wants to."
On Twitter, Jones chose not to share specifics of his illness.