no one really knows what precipitates diabetes -- and there's no true cure, only careful management -- but I've never heard any whisper of contagion
the effects are known, the systems involved are known, treatments are known
it's an internal thing, having to do with hormones, with internal organs, with genetics, and with diet, especially if it's Type 2, which is what our sugarcats seem to have
however, if it will make you feel better, you can scrub out the litterboxes with chlorine bleach (then rinse thoroughly) , or with white vinegar, or spray them with an enzyme cleaner made especially for cats
or get new ones
I can sympathize with how you're feeling, I lost Krispy cat to hepatic lipidosis, the vets tried but couldn't save him, and found I couldn't live without Vitamin P (purr)
which is why I adopted Catcat (and his companion Mia), soon afterwards -- they were being rehomed because of youngest son's asthma
they came with their own litterboxes and scratchpost ... that made it easy .. they didn't have to smell another cat in private places
no cat can be replaced, there are always more who need to be loved, each is an individual with their own habits and preferences
none of my pet cats will ever eat kibble again .. the barncats must but it's a minor part of their diet, they eat what they choose to catch, and the adjoining forest is a smorgasbord for them