Hepatitis D virus, also called hepatitis delta virus, is one of the strangest human viruses. Its genome is a tiny circular single-stranded RNA of only about 1,700 nucleotides, and it produces essentially one protein in two forms—the small and large hepatitis delta antigens. It does not encode its own enzyme to copy its RNA as most RNA viruses do. It doesn’t produce its own envelope proteins or most of the machinery normally associated with a virus. Instead, it has an extraord