Certain classes of viruses have acquired cellular genetic sequences as part of their evolutionary formation. The acquired sequences have essentially become stable components within the virus. The incorporation of cellular sequences can also occur more dynamically as part of an immune evasion mechanism termed stealth adaptation. This involves the loss or mutation of genes coding for the relatively few components that are normally targeted by the cellular immune system. It has been shown to occur with cytomegaloviruses infecting the types of monkeys in which cultured kidney cells were used to produce live polio vaccines. Genetic sequencing of polymerase chain reaction (PCR) products generated in a selection of these viruses has revealed rhesus monkey-derived cellular sequences. Human cellular sequences were present in an African green monkey-derived stealth adapted virus. The human sequences have presumably replaced African green monkey cellular sequences by homologous recombination. Stealth adapted viruses need to be viewed as carriers of potentially highly pathogenic, genetically unstable “renegade” cellular sequences. As discussed in other articles, microbe-derived renegade sequences can also be present in stealth adapted viruses. These findings open a new era in potential virus-induced illnesses.