Are the yellow spots on the old leaves only? If it was a virus, wouldn't the entire plant be affected, including flowers? Isolate it and see what develops, if anything. Use your sanitary practices (but you should use those as a matter of course).
If it is a virus, its not in the potting medium but in the plant tissue. Virus are spread by vectors in an exchange of bodily fluids - the same way we get viruses. That could be an insect but most likely a human. Wear disposable gloves and sterilize surfaces and tools between cuts and plants. A 10% bleach solution will kill the virus too, but it sure plays havic with your gardening tools.
