Humans and cats and dogs are different species and are not suceptible to the same diseases. Just like you don't get distemper or parvo. Most virual and bacterial illnesses are host specific, which means that they are caused by microorganisms that have evolved to attack one species and have been successful doing so. Dogs and cats are predators that have evolved from ancestors that ate old meat, so they have evolved to be resistant to some organisms that would make us very ill.
at 10:36 PM on Jun. 19, 2008