Adeno-associated viruses, computer illustration. Adeno-associated viruses (AAVs) are the smallest known viruses to infect humans. They do not cause diseases, and only provoke a mild immune response. Because they incorporate their genetic material into a specific location within the host's genome, they have potential as a vector for gene therapy. The spiky protein coat (capsid) protects the genetic material. The background shows a DNA autoradiogram with genetic code on a computer screen.