My WSD when she was about 18 months old or so caught parvo virus, she'd been vaccinated against it, and also had her yearly booster but still caught it. Luckily enough she had a cast iron constitution and made a full recovery. I've since been told by a vet that yearly vaccines aren't necessary, and being as my dog caught parvo when it was a relatively new virus to the doggy world I haven't had my dog vaccinated, or my cats. When I went away on holiday and my cats had to go into a cattery I went to the vets and they had their vaccinations so that they could go into the cattery but since then they've not been vaccinated (infact my elderly cat hadn't had any vaccinations until he had the ones for the cattery when he was 7 years old and he was a rescue cat). It could just be a case of vaccinations are better than they were years ago, more people have their animals vaccinated, all rescue centres vaccinate their animals before allowing them to be re-homed so the illnesses are naturally on the decline as it's not as easy to spread from dog to dog?