Sick Leave

We hear constantly about  people taking time off work sick, costing their employers additional expense and leaving their colleagues to pick up the slack.
At the moment, I know of two friends who are certainly not well enough to be in work. Yet both of them refuse to take any time off, citing guilt and the fear of job security as their reasons.
Are these attitudes, in actual fact more common than those of the so-called lazy work shy stereotypes?
Wishing them both a speedy recovery.