[giaour]

> And if they are clean and they presumably have a greater chance of a relapse than other random candidates.

That's skirting uncomfortably close to discriminating based on medical history or a disability, which may be illegal depending on where the employer is located. Cancer survivors have a much higher chance of recurrence than others do of developing cancer for the first time, but most would probably balk at denying a job to someone whose cancer was in remission because there's a higher chance they'd need extended medical leave in the future.