Why do some vaccinated animals still get the disease?

It is a fact that in the USA today, literally hundreds and perhaps thousands of vaccinated dogs and cats are still contracting the diseases they were vaccinated against. Some term this 'vaccine failure,' although it is more likely a failure of the immune system to respond than a problem with the vaccine itself.

Parvovirus is a serious case in point. How can a puppy get the disease and possibly die if it was vaccinated? Unfortunately, for some reason the vaccine did not stimulate the immune system enough to protect the puppy from disease. The reason may be interfering maternal antibodies, the vaccines themselves, the dog's own immune system, or genetics. By far, the most common reason in puppies is interfering maternal antibodies.