Some dogs are only mildly affected by fleas and they can cause irritation and itchiness to the skin. However, if left untreated the infestation of fleas can build up and the effects become more severe as your dog is being bitten by more fleas.
In some cases, dogs are allergic to the flea saliva and can have a severe reaction even if only bitten by one flea. This is known as Flea Allergic Dermatitis (FAD) and the skin can become very inflamed, scabby and even bald where the dog tries to rub and scratch.
Fleas also transmit some diseases like tapeworm and also Bartonella.
Fleas can also bite human’s ad cause an irritation and itchiness to the skin, however they will not live and stay on humans.