Adeno-associated virus. Coloured transmission electron micrograph (TEM) of particles of an unidentified adeno-associated virus (AAV, green). A helper adenovirus (orange) is also seen. Also known as dependoviruses, AAVs are defective viruses that are dependent on a co-infecting adenovirus for replication in their host's cells. AAVs can infect a wide range of vertebrate hosts without causing symptoms. However, they have no effect on the pathogenicity (disease-causing ability) of their helper viruses. Magnification unknown.