The viruses that are associated with CFS are termed "stealth" because they can cause significant cellular damage, yet do not typically evoke an anti-viral inflammatory response. They are best viewed as "derivatives" or as "down-sized" conventional viruses with added cellular genetic components. They include but are not limited to derivatives of herpesviruses, adenoviruses, papovaviruses and probably enteroviruses. The "stealth adaptation" consists primarily of the deletion of the genes coding for the major antigenic components normally targeted by the cellular immune system. Stealth viruses do not grow as efficiently as conventional viruses, but have a striking advantage over conventional viruses in not having to confront the body's cellular immune defense mechanisms. They can, therefore, create persistent ongoing infections in spite of an individual's intact immune system. This is different from a latent infection seen with many human herpesviruses in which the virus is essentially inactive except for brief transient periods of viral activation; rapidly controlled by the body's immune mechanisms.