Multiple levels are also optimum for multiple cat households because they don't have to fight over spots, there room for all of them. (thoroughlyreviewed.com)
It moves between cats through stool - contamination; so the litter box in multiple cat households is a good place to find the virus. (2ndchance.info)
The instructions here are written with multiple cats in mind, but apply equally, if you're only dealing with one or two cats. (feralcatcaretakers.org)