What can I use on a five week old puppy infested with fleas? I don't want to bring her into my home that way. I don't have other pets.

The pup is a little young for over the counter stuff . I would talk to your vet.

Is the puppy from a shelter or a breeder or you found her?

If she is from a breeder then is not allowed to be sold under 8 weeks of age so tell the breeder what they are doing is against the law and get them to fix it.

If she is from a shelter, they will rid the fleas before you take her.

And finally if you found her then she will need a vet visit anyway to make sure she does not have any medical problems and to de-worm her.