What is it about the other crops, planted in close proximity, that protect it from CMV?
The main vector for the spread of CMV is aphids. When an aphid feeds on a kava plant infected with CMV, some of the virus can stick around in its mouthparts and be carried to the next plant it feeds on. If that next plant is also a CMV host, it can be infected with the virus.
In feeding again on a plant that didn't already have CMV, its mouthparts are cleaned of the virus. So any further feeding after that does not spread the virus until it feeds on another CMV-infected plant.
Basically, aphids normally only have the ability to spread the virus to the first plant it feeds on after feeding on an infected plant. So you want to make sure that you interplant your kava with plants that aren't CMV hosts, and when you do spot an infected plant, remove it immediately.
@Alia will know a more about this than I. AFAIK a lot of this information came from a brilliant plant pathologist in Hawaii, Scott Nelson.