hello, I've got some questions about dog fleas...

Are they the same as cat fleas so can a dog catch fleas off a cat? If a dog has fleas but shows no signs of a problem eg. skin is ok, no scratching, dog seems happy and healthy etc. does it actually do the dog any harm to have them? If a dog or cat has fleas for a long time untreated, is it fair to assume they are also living in the house, and that it would take more than just hoovering the carpet to get rid of them?

Sorry for so many questions, any help appreciated. x