It would be interesting, but perhaps not terribly useful, to see how sickness absence varies between contract and permanent staff. In the past, when contracting, I have dragged myself to work when feeling dreadful.
I've been contracting for the last 13 years. I've had a few occasions where I feel I "worked through" a period of illness too enthusiastically and ended up making myself feel worse and lengthening the illness. Knowing that you need to bill, and therefore need to work, is tough to balance against stopping work to get better. It's been a tough lesson for me, but I try very hard to slow down or even stop work when I'm sick.
On the last occasion I really tried to "power through" an illness (with a persistent bad cough and even more persistent sore throat) I started thinking about Jim Henson's death. That made me slow down. Nothing is worth that.