Many years ago, I had a friend and coworker who called in sick at the drop of a hat (I was in the securities industry at the time.) When admonished by management for it, she complained to the rest of us that "it isn't my fault that I'm the kind of person who gets sick easily" (actually, given her lifestyle, it probably was her fault, but that's beside the point.) I explained to her that when she was hired, it was because there was a set of tasks that needed to be done. These tasks still need to be done whether or not it is her fault she's sick. If she isn't there, it falls to her coworkers to get these things done. This is not fair to the rest of the team. If you're truly that chronically ill, you should be on disability.
She didn't get it. Continued to call in sick, we continued to pick up the slack.