Contagious? Of course not. I have 5 cats & none are diabetic. Know many people with diabetic cats & the people don't have diabetes.
More & more dogs & cats are being diagnosed diabetic from being fed grain based, high carb crappy supermarket food. Dogs & cats don't have ability to digest grains & would never be eating this in the wild. Cats are carnivores & dry food is the worst thing for them. Another issue with cats stressing out their pancreas is being free-fed. A cat's digestive system is triggered by smell. Leaving food out all the time keeps their digestive system turned on.
Cats have an unusual form because it can go into remission & then they don't need injected insulin.