Viruses are masters at delivering genetic material to the nucleus of the organisms they infect, and because of this, they were the first vectors that were developed for gene therapy delivery. A plethora of viral vectors entered the gene therapy arena including lentiviruses, retroviruses, herpes simplex viruses, adenoviruses and adeno-associated viruses. Each of these vector candidates are highly immunogenic, recognized quickly by the immune system and cleared from circulation – leaving antibodies specific for these viruses surveilling the body for the next potential infection.