How would one clean a mattress and rid it of old fusty body odour?

I worked in the family used furniture store for a brief stint so I can help some. Do a good cleaning as the other answer stated then mix up a bleach & water solution, 1 part bleach to 10 parts water, and spray it all over the mattress until it's damp. Let dry well and then do the other side too. That 10:1 mix will also kill most bacteria, germs, fungus, virus, etc. too. Don't worry, the bleach will not fade any of the color either, I've used it full strength and the colors stayed. If the odors are down in the cotton/foam padding then you'll need to replace the mattress, but I doubt anything got down that far, those mattress covers are very tough & tight woven.