To NOT vaccinate dogs...is there a real threat or peta propaganda?

I've heard of people doing this, but it's not animal rights (at least it wasn't when I heard them doing it) it was about being natural and holistic with their dogs, no chemicals at all. A lot of people don't agree with it, but I still know people who don't vaccinate. I also know people who only vaccinate puppies, but not adults, they feel it's dangerous.