The incidences are so low and if you practice proper barrier precautions the risks are even lower. I dunno the exact percentages but at some point you're talking chance of getting struck by lightning or contracting HIV from a random needle stick.
I've done at least two CJD taps in the last couple of years and the keys are technique and a sedated patient.
If you're really worried about the contamination stuff, stop putting your bag in the work room next door and wear booties in the area. Is there even evidence to suggest disease transmission from random environmental contact?