advances in genome sequencing technology and oligonucleotide synthesis paved the way for construction of synthetic genomes based on previously sequenced genomes. both rna and dna viruses can be made using existing methods. rna viruses have historically been utilized due to the typically small genome size and existing reverse transcription machinery present. the first man-made infectious viruses generated without any natural template were of the polio virus and the φx174 bacteriophage. with synthetic live viruses, it is not whole viruses that are synthesized but rather their genome at first, both in the case of dna and rna viruses. for many viruses, viral rna is infectious when introduced into a cell (during infection or after reverse transcription). these organisms are able to sustain an infectious life cycle upon introduction "in vivo".