If your dog is scratching, you should check for fleas as well as tapeworms (from accidentally eating fleas that carry tapeworm eggs). In puppies, it can cause anemia and can cause death within a few hours—press upper gum, if it stays white rather than returning to pink your puppy may have anemia, act quickly. While there are cat and dog fleas, the cat flea is found more commonly in the US among dogs, cats and people, while the dog flea is found primarily in Europe. When the temperature is 65 and the humidity is 75 , fleas are in season. While tiny, you can see them or “flea dirt” which looks like dark specks.
Unfortunately, if your dog has fleas, it is likely that you may have fleas as well in your home or yard. You will need to remove them from all areas. There are prescription products for your pet that control the hatching of the eggs which can be effective but you will also need to address your home and yard (if you have a lot of fleas). Ask your vet what they recommend especially if your dog is pregnant, nursing or is a puppy under 10 weeks.
There are four stages of fleas: eggs, larvae, pupae and adults and the total of these stages last 14 to 75 days. They can live for a year as fully formed larvae until conditions are right for them to emerge (heat, vibrations and exhaled carbon dioxide). Adult fleas, that live 4-25 days, require fresh blood to produce eggs and that is when they live on your pet (and maybe you).