"OK, you're dealing with an illness, and in some way, shape or form there's laws against that and there's laws about it," Fox said.
"I mean I'm not being evasive, but I kind of am. Yeah. And for the right reason. Yeah."
"Uh, I think he was tired today," Fox said. "[That] was what I heard."
Sims hasn't practiced since the bye week, but he's been spotted inside the team facility in close proximity to teammates.
Fox was asked to explain why Sims is allowed in Halas Hall if he's so sick.
"They're not worried about that part right now," Fox said. "I'm not the doctor here. All right? Yeah. I wish I could be, but they won't let me do that. I would get in trouble."
The Bears signed Sims to a three-year deal in the offseason that paid him $6 million guaranteed in 2017. Sims has nine catches for 113 yards and one touchdown.
Also, right guard Kyle Long (fingers), cornerback Sherrick McManis (hamstring) and defensive end Roy Robertson-Harris (hamstring) were all upgraded to full participation in practice.
Long, who is also dealing with ankle and shoulder injuries, was active last week but played just one snap on special teams.
"I joke with our trainer sometimes," Fox said. "He goes, 'Well, [Long] feels pretty good.' I said, 'Yeah, he's been out for two weeks. He ought to feel good.' All right? So I think that helps, you know, rest. You had the bye week, and really we didn't use him extensively last week against Green Bay, so I think he feels better, and he looked good in practice."