Most puppies have some immunity from their mothers, and it tends to affect dogs in rescue kennels as they are usually from unvaccinated parents and are in an environment where there is likely to be lots of dogs in confined areas and very stressful surroundings. The stories put out by vets every so often about outbreaks are usually concerning rescues and I believe (perhaps cynically) that they are meant to scare people into vaccinating. Personally I don't vaccinate adult dogs and there is much evidence to suggest that immunity from puppy vaccines last a long time if not a lifetime. Puppies that do succomb do not usually survive unless caught early and if they do they tend to have problems in later life such as a weak heart. When it first came to light years ago many greyhound kennels were affected and even if the dogs survived they would often collapse when training or racing.