Adeno-associated virus capsid, molecular model. The capsid is a protein shell that encloses the virus' genetic information. Adeno-associated viruses (AAVs) are the smallest known viruses to infect humans. This is serotype 2 of the virus. AAVs 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.