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.
That the situation one of my college buddies (and all his fellow contractors) endured for a couple years. They did everything the normal workers did, but since they were technically "contractors" in title, everyone treated them like second-class citizens. (Caveat: Major japanese company--but still in the USA.)
If that's not the case with yours, I'm glad they're not in the same situation. My friend eventually moved up to one of the top contracters and still couldn't get into the company proper so he had to seek employment elsewhere.